Paragraph 5: The Jovian planets have very thick atmospheres consisting of
varying amounts of hydrogen, helium, methane, and ammonia. By comparison,
the terrestrial planets have [meager] atmospheres at best. A planet's ability
to retain an atmosphere depends on its temperature and mass. Simply stated,
a gas molecule can evaporate from a planet if it reaches a speed known as
the escape velocity. For Earth, this velocity is 11 kilometers per second.
Any material, including a rocket, must reach this speed before it can leave
Earth and go into space. The Jovian planets, because of their greater masses
and thus higher surface gravities, have higher escape velocities (21-60
kilometers per second) than the terrestrial planets. Consequently, it is more
difficult for gases to "evaporate" from them. Also, because the molecular
motion of a gas depends on temperature, at the low temperatures of the Jovian
planets even the lightest gases are unlikely to acquire the speed needed to
escape. On the other hand, a comparatively warm body with a small surface
gravity, like Earth's moon, is unable to hold even the heaviest gas and thus
lacks an atmosphere. The slightly larger terrestrial planets Earth, Venus,
and Mars retain some heavy gases like carbon dioxide, but even their
atmospheres make up only an infinitesimally small portion of their total mass.
5. The word [meager] in the passage is closest in meaning to
○over time
○long ago
○simply
○certainly
答案是B 不懂怎麼選的耶......
又要請戰友們幫忙了 快考試啦～好緊張
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◆ From: 174.1.41.12
※ 編輯: Diablotin 來自: 174.1.41.12 (06/13 06:12)