(A)

We have always been interested in the moon. 2000 years ago people already knew it moved around the earth and where it would be in the sky at different times of the year. At that time, everything about the moon was learned by watching it carefully in the sky.
　　When scientists could use telescopes to study the moon more closely, their ideas began to change. They could see the moon was made of rocks. Most scientists thought moon rocks would be different from those on Earth. This was because they believed the moon had once been a planet that had been caught in the earth's gravity (引力) millions of years earlier.
　　In 1969 moon rocks were finally brought to the earth and studied. Much to their surprise, scientists found that, except for water, the moon and the earth were made of the same things. Once again new ideas were needed for this new information.
　　After years of study, most scientists now think that the moon was once part of Earth. They believe very early in its history, maybe 4 million years ago, something about the size of Mars hit Earth. This sent billions of rocks into space around our planet. These rocks slowly joined together and after many years became the moon.
    In the future, even though our ideas about the moon may change again, we will still be interested in it.

(  )51. Now, we’re sure ________________

A. Moon rocks are quite different from those on Earth.

B. Moon was once a part of Earth.

C. Moon moves around Earth and it’s the only satellite(卫星) of Earth.

D. Moon was once a planet caught in Earth’s gravity.

(  )52. In 1969 there is a big event, that is __________.

A. humans found water in Moon rocks      B. scientists made out a modern spaceship

C. scientists invented an advanced telescope   D. humans left their footprints on Moon