(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

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