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  3. As viewed from space, the Earth’s distinguishing characteristics are its blu

As viewed from space, the Earth’s distinguishing characteristics are its blu

游客2025-02-03  1
问题     As viewed from space, the Earth’s distinguishing characteristics are its blue waters and white clouds. Enveloped by an ocean of air consisting of 78% nitrogen and 21% oxygen, the planet is the only one in our solar system known to harbor life. Circling the Sun at an average distance of 149 million km (93 million miles), the Earth is the third planet from the Sun and the fifth largest planet in the solar system. Its rapid spin and molten nickel-iron core give rise to an extensive magnetic field which, coupled with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Most meteors burn up in the Earth’s atmosphere before they can strike the surface. The planet’s active geological processes have left no evidence of the ancient pelting it almost certainly received soon after it was formed. The Earth has a single natural satellite -- the Moon. [br] Approximately how much of the Earth’s atmosphere is nitrogen?
选项 A、One-fourth.
B、One-half.
C、Three-fourths.
D、All of it.
答案 C
解析 见文中第二句话：Enveloped by an ocean of air consisting of 78% nitrogen and 21% oxygen，the planet is the only one in our solar system known to harbor life．地球由内含78%氮气和21%氧气的大气层包裹，是太阳系中唯一一个已知能够含有生命的星球。其中78%的氮气符合选项“地球大气中氮气含量约占四分之三”。
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