2. 外语
  3. Many years ago people thought the earth was flat. They believed that the sky【C1】______a big upside-down bowl and that the sun, m

Many years ago people thought the earth was flat. They believed that the sky【C1】______a big upside-down bowl and that the sun, m

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问题     Many years ago people thought the earth was flat. They believed that the sky【C1】______a big upside-down bowl and that the sun, moon, and stars were lamps hanging from the top of the bowl. Later, people【C2】______better. They found out that the earth【C3】______round.
    We call the study of the skies and the planets astronomy, 【C4】______we call the watchers of the skies astronomer. Early astronomers thought the earth was the center of【C5】______They thought the heavens spun【C6】______the earth. Later, they changed their【C7】______. They knew the earth was not the center. They believed that the earth and the【C8】______planets spin around the sun. Today we know this is true.
    Ancient scientists studied the skies【C9】______simple instruments. The first person【C10】______a telescope was named Galileo. In 1610【C11】______four moons around the planet Jupiter. Today【C12】______use more complex equipment. Some of these measure the brightness of light. 【C13】______break lights up into different colors. Radio telescopes make stars and planets【C14】______far closer than Galileo’s telescope could.
    Early astronomers【C15】______five planets in the sky. Besides Earth, they knew about mercury, Venus, Mars, Jupiter, and Saturn. In 1781 a seventh one was discovered and named Uranus. Years later, astronomers found another planet and called it Neptune. The ninth planet to be found is called Pluto. It was not【C16】______until 1930.
    Today scientists can【C17】______almost anywhere. They have reached the heavens with their spacecraft. In 1959, a spacecraft【C18】______took pictures of the dark side of the moon. The years later a spacecraft landed two Americans on the moon. Other important journeys into space【C19】______. Who knows? Maybe【C20】______human beings will live on another planet. Maybe you will be one of them.
【C15】
选项 A、could have seen
B、could see
C、had seen
D、saw
答案A
解析 空格处对过去的某种状态进行推测。因为在讲述过去的某种状态，用现在完成时即可。因为没有在过去某点说过去保持的某种状态，因此，不使用过去完成时。因为早期天文学家知道包括地球在内的六大行星，因此，他们可能已经观测到了其他五大行星，但没有证据予以证实，因此，用could表示推测，更符合原文意思。因此，A为正确选项。
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