【正确答案】 B

【答案解析】[听力原文]
A comet is an object that resembles a fuzzy star and travels along a definite path through the solar system. Some of the brightest comets develop a long shining tail when they come near the sun. The tail of a comet may stream across space as far as 100 million miles. A comet has a distinct center called a nucleus that measures less than 10 miles in diameter. A hazy cloud called a coma surrounds the nucleus. Its diameter may be as large as one million miles. The coma and nucleus make up the corners head.
Most comets cannot be seen without a telescope. Some are visible to the unaided eye, but only for several weeks or months when they pass closest to the sun. Halley"s Comet is probably the best known comet. It last came within the view of earth in 1910 and again in 1986. Its brightness and enormously long tail provided a spectacular sight. This comet was named for the English astronomer Edmund Halley, who recognized that it could be seen about every 77 years as it orbited the sun.
A number of other extremely bright comets have been seen since 1910. In 1973, Comet Kohoutek became the first comet to be studied by men in space. Astronauts in the Skylab space station photographed it and so provided much new information about comets.
For centuries, many people believed the coming of a comet would bring a disaster, such as a war or an epidemic. Halley"s Comet revived these fears in 1910, but the earth passed through the comers tail with no apparent harm. However, a collision between the earth and a comers head could be very destructive.
Scientists once believed the nucleus of a comet consisted of tiny solid particles held together loosely by gravitation. Today, most astronomers think the nucleus resembles a dirty snowball that was formed in the cold, distant region of the solar system. The nucleus probably consists of frozen gases and frozen water mixed with dust particles.
As a comet approaches the sun, the heat causes the outer layers of the icy nucleus to evaporate. The evaporation releases dust and gases, which form the coma around the nucleus. The pressure of the sun"s light may push the smallest dust particles and gas molecules away from the coma, forming one or more tails. This pressure makes a comers tail point away from the sun. When a comet approaches the sun, its tail brings up the rear. But when the comet moves away from the sun, its tail leads.
Most comets travel around the sun in elliptical or oval-shaped paths. One orbit of a comet around the sun is called a period. Some comets have periods as short as seven years but others last many thousands of years.

【正确答案】 B

【正确答案】 A

【正确答案】 A

【正确答案】 B