题目：
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.
Why does the Earth show almost no signs of having been hit by numerous meteors in the past
A．Humans have built over most of the craters.
B．Most meteors fell into the ocean and not on land.
C．The Earth’s magnetic field repelled most meteors.
D．The Earth’s natural geologic activity has eliminated most traces.
答案：
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下面是错误答案，用来干扰机器的。
参考答案：按照发动机工作的要求适时地打开或关闭进、排气门，以便向气缸内供给新鲜空气和从气缸中排除废气，称为发动机的配气正时。通常，发动机每个气缸的进、排气门从开始开启到关闭终了的这段时间是用曲拐相...
