Questions 10-19 　　No two comets ever look identical, but they have basic features in common, one of the 　　most obvious of which is a coma. A coma looks like a misty, patch of light with one or more 　　tails often streaming from it in the direction away from the Sun. 　　Line At the heart of a comet's coma lies a nucleus of solid material, typically no more than 　　(5) 10 kilometers across. The visible coma is a huge cloud of gas and dust that has escaped 　　from the nucleus, which it then surrounds like an extended atmosphere. The coma can extend as far as a million kilometers outward from the nucleus. Around the coma there is often an even larger invisible envelope of hydrogen gas. 　　The most graphic proof that the grand spectacle of a comet develops from a relatively 　　(10) small and inconspicuous chunk of ice and dust was the close-up image obtained in 1986 by the European Giotto probe of the nucleus of Halley's Comet. It turned out to be a bit like a 　　very dark asteroid, measuring 16 by 8 kilometers. Ices have evaporated from its outer layers 　　to leave a crust of nearly black dust all over the surface. Bright jets of gas from evaporating 　　ice burst out on the side facing the Sun, where the surface gets heated up, carrying dust 　　(15) with them. This is how the coma and the tails are created. 　　Comets grow tails only when they get warm enough for ice and dust to boil off. As a 　　comet's orbit brings it closer to the Sun, first the coma grows, then two distinct tails usually 　　form. One, the less common kind, contains electrically charged (i.e., ionized) atoms of gas, 　　which are blown off directly in the direction away from the Sun by the magnetic field of 　　(20) the solar wind. The other tail is made of neutral dust particles, which get gently pushed back by the pressure of the sunlight itself. Unlike the ion tail, which is straight, the dust tail 　　becomes curved as the particles follow their own orbits around the Sun. 　　I0. The passage focuses on comets primaril
ly in terms of their 　　(A) orbital patterns 　　(B) coma and tails 　　(C) brightness 　　(D) size 　　11. The word "identical" in line I is closest in meaning to 　　(A) equally fast 　　(B) exactly alike 　　(C) near each other 　　(D) invisible 　　- 　　12. The word "heart" in line 4 is closest in meaning to 　　(A) center 　　(B) edge 　　(C) tail 　　(D) beginning 　　13. Why does the author mention the Giotto probe in paragraph 3? 　　(A) It had a relatively small and inconspicuous nucleus. 　　(B) It was very similar to an asteroid. 　　(C) It was covered with an unusual black dust. 　　(D) It provided visual evidence of the makeup of a comet's nucleus. 　　14. It can be inferred from the passage that the nucleus of a comet is made up of 　　(A) dust and gas 　　(B) ice and dust 　　(C) hydrogen gas 　　(D) electrically charged atoms 　　15. The word "graphic" in line 9 is closest in meaning to 　　(A) mathematical 　　(B) popular 　　(C) unusual 　　(D) vivid 　　16. Which of the following occurred as the ices from Halley's Comet evaporated? 　　(A) Black dust was left on the comet's surface. 　　(B) The nucleus of the comet expanded. 　　(C) The tail of the comet straightened out. 　　(D) Jets of gas caused the comet to increase its speed. 　　17. All of the following statements about the tails of comets are true EXCEPT: 　　(A) They can contain electrically charged or neutral particles. 　　(B) They can be formed only when there is sufficient heat. 　　(C) They are formed before the coma expands. 　　(D) They always point in the direction away from the Sun. 　　18. The word "distinct" in line 17 is closest in meaning to 　　(A) visible 　　(B) gaseous 　　(C) separate 　　(D) new 　　19. Compared to the tail of electrically charged atoms, the tail of neutral dust particles is 　　relatively 　　(A) long 　　(B) curved 　　(C) unpredictable 　　(D) bright