D

(This selection was originally published in 1992.Pluto (冥王星) is no longer classified as a planet.)

Pluto is in many ways the strangest of the planets.It is small and has a large moon (called Charon).Its orbit is unusual, which may cause it to have seasons in the sense that when it is close to the sun, the liquid methane(甲烷) on its surface boils to form a kind of atmospheric haze (烟雾). When the planet moves farther away from the sun, it starts to snow solid methane.

Pluto is not dark.Despite its great distance from the sun, the surface of Pluto is probably as bright as a moonlit night on Earth.The reason is all that methane, which is as white as newly fallen snow.

The discovery of Pluto was more accident than design.The American astronomer Percival Lowell had predicted the existence of a ninth planet (he called it Planet X) based on what he took to be irregularities in the orbit of Neptune.Today astronomers argue that these "irregularities" weren't real, but the result of instrumental error.Nevertheless, Lowell produced predictions about where Planet X ought to be (although, to be honest, the predictions changed occasionally when he redid the calculations).In any case, in 1930 Clyde Tombaugh, doing a systemic sky survey that would have found the planet no matter where it was, discovered the planet we now call Pluto.By coincidence, its position was pretty close to where Lowell's last prediction said it should be.Was it just luck? We'll never know.

68.What contributes to the brightness of Pluto?

A.Light from its moon, Charon. B.Its great distance from the sun.

C.Methane on its surface. D.Its closeness to the sun.

69.On Pluto's surface, liquid methane boils to form an atmospheric haze, which would be most similar to ____.

A.darkness B.light

C.liquid D.steam

70.What effect is produced by including the question "Was it just luck?" at the end of the selection?

A.The question encourages the reader to think about why Pluto is a strange planet.

B.The question encourages the reader to think about how planets are discovered.

C.The question suggests that other planets were found in different ways.

D.The question suggests that all discoveries are luck, not just design.

71.The selection mainly wants to ____.

A.give a brief introduction of Pluto B.talk about the discovery of Pluto

C.prove the exact position of Pluto D.describe the strangeness of Pluto