单选题 Comets are the most oddly behaved objects in the sky. No two of them act exactly alike. Most appear without warning, seemingly out of nowhere, too faint at first to be detected except as fuzzy dots of light on the photographic plates of automatic cameras attached to telescope lenses; most of the members of the comet family move in elliptical paths, remain visible to earthly observers for a few weeks or months, then disappear into the depths of space. There are a few comets that return periodically, on predictable timetables following almost the same track they were on originally. But even those few have little in common. The tracks traveled by some of them must extend very far away from the sun, for decades pass between their appearances. Other comets come back at intervals as short as three to four years. Halley"s comet (named after the British astronomer Edmund Halley, who predicted its return in 1758) was seen, with a single exception, every seventy-seven year from 240 BC to 1910 and is expected to return again in 1987. It should be noted that one of the few, characteristics shared by all of the 1700 comets observed since 2316 Be is the common focal point of their elliptical orbits—the sun.
Though most comets are too small to be measured accurately, some are enormous. The great comet of 1843 had a tail twice as long as the distance from the earth to the sun. The head of the comet of 1811 was alone bigger than the sun. The heads of some comets are composed of a bright nucleus shrouded by a nebulous coma; in the heads of other comets, no nucleus can be seen. The coma may or may not have a tail. Again, one of the few similarities among comets must be remarked on. Where a nucleus is present, discharges of some kind usually stream from it into the coma and the tail.
Planet earth passed through the tail of Halley"s comet in 1910, while the comet head was 15,000,000 miles away. Despite its giant size, the comet did not contain enough mass to exert any noticeable gravitational pull on earth. Brooks comet in 1866 passed between the satellites of planet Jupiter and Jupiter itself without causing the slightest perturbation in the orbits of the satellites, although the comet"s own orbit was shortened from twenty-seven years to seven. By contrasting these experiences, it is seen that comets are by earthly measure insubstantial stuff. It is no surprise then that spectrographic examination of the light they emit shows comets to be molecular mixtures of frozen gases—principally hydrogen, oxygen, nitrogen, and carbon—and cosmic dust.
As recently as the first half of the twentieth century, it was believed that the luminosity of comets rose solely from the reflected light of the sun. Subsequently, astronomers have determined that comets also shine with intrinsic light, perhaps triggered somehow by the sun. In searching for a possible triggering mechanism, it is first desirable to draw together, from the scientific literature on comets, descriptions of erratic fluctuations in comet light.

单选题 Most comets ______.

单选题 The tails of comets are ______.

单选题 What would probably happen if a comet should pass a planet?

单选题 Comets ______.

单选题 Which of the following is TRUE?