单选题《复合题被拆开情况》 In contrast to the political anarchy, economic dislocation, and military destruction in Spanish America, Brazil’s drive toward independence from Portugal proceeded as a relatively bloodless
单选题《复合题被拆开情况》 METAMORPHOSIS 1 Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatches a tadpole that metamorphoses into an adult frog within a few days
单选题《复合题被拆开情况》Why does the man need the professor’s help?《问题》：What is the cause of the man’s problem?
单选题《复合题被拆开情况》 METAMORPHOSIS 1 Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatches a tadpole that metamorphoses into an adult frog within a few days
单选题《复合题被拆开情况》 It is now well-established that petroleum migrates through aquifers and can become trapped in reservoirs. Petroleum migration is analogous to groundwater migration. When oil and gas are squ
单选题《复合题被拆开情况》 COMETS 1 Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases water vapor, ammonia, methane, carbon dioxide, and carbon monoxide
单选题Why does the man need the professor’s help?
单选题《复合题被拆开情况》 MESOLITHIC COMPLEXITY IN SCANDINAVIA 1The European Mesolithic roughly the period from 8000 B.C. to 2700 B.C. testifies to a continuity in human culture from the times of the Ice Age. [A] Th
单选题. Neutron Stars While the largest star known to man is roughly 2,000 times the diameter of the Earth's sun, there are also stars so tiny that they are no larger than a small metropolitan area. These are neutron stars. A neutron star is extremely dense and is essentially the corpse of a giant star which collapsed after going supernova. Despite its small size, a neutron star has a mass greater than one and a half times that of the sun, is among the hottest stars in the galaxy, and has one of the strongest magnetic fields of any object in existence. Whenever a neutron star forms, it begins spinning rapidly, which causes the electromagnetic energy it emits to appear to blink, or pulse, to observers on the Earth. Astronomers call these spinning, blinking neutron stars pulsars. A star must reach a certain size—roughly four to eight times the size of the sun—to be able to form a neutron star. Stars smaller than that typically become white dwarfs when they collapse while those larger than that normally transform into black holes upon collapsing. The formation of a neutron star occurs when a supernova destroys a star's outer layer yet leaves the dense core intact. The center of the star is usually comprised of iron, but other elements may be present in smaller amounts. With the star's outer layers gone, nuclear fusion is no longer possible, which results in gravity having no counterbalancing outward force. Therefore, gravity acts strongly on the core and makes it extremely dense. As this density grows in strength, the electrons and protons in the core's atoms are forced together to form a core of neutrons and neutrinos. The neutron star forms a structure with different layers, much like other bodies in the universe. If the neutron star is relatively young, it is still very hot, so its surface may be more liquid than solid while its inner core is much denser and more solidified. A neutron star's temperature is at its height soon after its birth, but the star gradually begins cooling off as it becomes older on account of the loss of neutrinos, which start flying off the star as soon as it is created. Over time, these neutrinos carry away a great amount of the star's energy, so its temperature falls to a level approximately half of what it was at its genesis. Nevertheless, even after this loss of temperature, a neutron star ranks as one of the hottest stars in the universe. In addition, when it forms, a neutron star spins at an extremely high rate—somewhere between sixty and 600 times per second. This high rotation is explained by the law of conservation of angular momentum, which points out that an object rotates at a faster rate the more it is drawn in on itself. This is similar to the way that a figure skater begins spinning faster the moment that she pulls her arms toward her body. The extreme density of a neutron star creates a very strong electromagnetic field, which is most often observed in the X-ray band. This magnetic field is aligned along a different axis than the spinning motion of the star, so it is like a lighthouse beam rotating in space. If the star is aligned in the right way with the Earth, astronomers using radio telescope arrays can observe this spinning magnetic field. Astronomer Jocelyn Bell first observed this phenomenon in 1967, and such stars were soon called pulsars. Most pulsars emit electromagnetic energy in the X-ray band, but some emit it in the visible light band as well as the gamma ray band. These observations led to the discovery of more neutron stars—almost all of them pulsars—in the following decades. Some are called millisecond pulsars because of the fact that they rotate as many as 700 times per second. At present, astronomers have discovered nearly 2,000 neutron stars; however, it is believed that the number of neutron stars is far greater since all of them cannot be observed from the Earth because their rotating magnetic fields are misaligned with the planet. In addition, as neutron stars age, the speed of their rotation decelerates, preventing them from being discovered as easily as more swiftly rotating neutron stars. *Glossary nuclear fusion: a reaction in which the nuclei of light atoms unite to form the nuclei of heavier atoms neutrino: a massless or nearly massless lepton, an elementary type of particle millisecond: one thousandth of a second28. Neutron Stars While the largest star known to man is roughly 2,000 times the diameter of the Earth's sun, there are also stars so tiny that they are no larger than a small metropolitan area. These are neutron stars. A neutron star is extremely dense and is essentially the corpse of a giant star which collapsed after going supernova. Despite its small size, a neutron star has a mass greater than one and a half times that of the sun, is among the hottest stars in the galaxy, and has one of the strongest magnetic fields of any object in existence. Whenever a neutron star forms, it begins spinning rapidly, which causes the electromagnetic energy it emits to appear to blink, or pulse, to observers on the Earth. Astronomers call these spinning, blinking neutron stars pulsars. The word corpse in the passage is closest in meaning to ______
单选题《复合题被拆开情况》 COMETS 1 Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases water vapor, ammonia, methane, carbon dioxide, and carbon monoxide
单选题《复合题被拆开情况》 WHAT CONTROLS FLOWERING 1 The timing of flowering and seed production is precisely tuned to a plant’s physiology and the rigors of its environment. In temperate climates, plants lost flower
单选题《复合题被拆开情况》 MESOLITHIC COMPLEXITY IN SCANDINAVIA 1The European Mesolithic roughly the period from 8000 B.C. to 2700 B.C. testifies to a continuity in human culture from the times of the Ice Age. [A] Th
单选题《复合题被拆开情况》 WHAT CONTROLS FLOWERING 1 The timing of flowering and seed production is precisely tuned to a plant’s physiology and the rigors of its environment. In temperate climates, plants lost flower
单选题《复合题被拆开情况》 FOREST SUCCESSION—LAYERS OF A FOREST 1 Succession is a continuous change in the species composition, structure, and function of a forest through time following a disturbance. Each stage of
单选题 It is usually claimed that species-rich ecosystems tend to be more stable than species-poor ecosystems. Three mechanisms by which higher diversity increases ecosystem stability have been proposed. Fi
单选题《复合题被拆开情况》 In contrast to the political anarchy, economic dislocation, and military destruction in Spanish America, Brazil’s drive toward independence from Portugal proceeded as a relatively bloodless
单选题《复合题被拆开情况》 FOREST SUCCESSION—LAYERS OF A FOREST 1 Succession is a continuous change in the species composition, structure, and function of a forest through time following a disturbance. Each stage of
单选题Why does the professor mention books and a map?
单选题 Although literacy appeared independently in several parts of the prehistoric world, the earliest evidence of writing is the cuneiform Sumerian script on the clay tablets of ancient Mesopotamia, which