HOW LIFE BEGAN ON THE EARTH
No one knows (1)_______(exact) how the earth began as it happened so long ago. However according to a widely (2) _______(accept) theory the universe began with a “Big Bang” (3)_____ threw matter in all directions. After that atoms began to form and combine (4)________(create) stars and other bodies.
For several billion years after the “Big Bang” the earth was still just a cloud of dust. (5)______it was to become was uncertain until between 4.5 (6)_____ 3.8 billion years ago (7)______ the dust settled into a solid globe. The earth became so violent (8)_______ it was not clear (9)________ the shape would last or not. It exploded loudly with fire and rock. They were in time to produce carbon nitrogen water vapor and other gases (10)_______ were to make the earth’s atmosphere. What is even more important is (11)______ as the earth cooled down water began to appear on (12) _____(it) surface.
Water had also appeared on other planets like Mars but (13)______(like) the earth it had disappeared later. It was not immediately obvious that water was to be fundamental (14)______ the development of life. What many scientists believe is that the continued (15)_________(present) of water allowed the earth to dissolve (16)________(harm) gases and acids into the oceans and seas. This produced a chain reaction which made (17)_____ possible for life to develop.
Many millions of years later the first (18)_________(extreme) small plants began to appear on the surface of the water. They multiplied and filled the oceans and seas (19)______ oxygen which encouraged the later development of early shellfish and all sorts of fish. Next green plants began to grow on land. They were followed in time (20)_____ land animals. Some were insects. Others (21)_______(call) amphibians were able to live on land as well as in the water. Later when the plants grew into forests reptiles appeared (22)_____ the first time. They produced young generally by (23)______(lay) eggs. After that some huge animals called dinosaurs developed. They (24)_____(lay) eggs too and existed on the earth for more than 140 million years. (25)_________ 65 million years ago the age of the dinosaurs ended. Why they suddenly disappeared still remains a mystery. This (26)___________(appear) made possible the rise of mammals on the earth. These animals were different from all life forms in the past because they gave birth (27)_____ young baby animals and produced milk (28) _______(feed) them.
Finally about 2.6 million years ago some small clever animals now (29)_____ hands and feet appeared and spread all over the earth. Thus they have in (30) ______ (they) turn become the most important animals on the planet. But they are not looking after the earth very well. They are putting too much carbon dioxide into the atmosphere which prevents heat from (31)_________(escape) from the earth into space. As a result (32)____ this many scientists believe the earth may become too hot to live on. So whether life will continue on the earth for millions of years to come will depend (33)_____ whether this problem can (34)_________(solve).
A visit to the moon
Last month I was lucky enough to have a chance (1) _______(make) a trip into space with my friend Li Yanping an astronomer. We visited the moon in our spaceship!
Before we leave Li Yanping explained (2)____ me that the force of the gravity would change three times on our journey and (3) ______ the first change would be the most powerful. Then we were off. (4)_____ the rocket rose into the air we were pushed back into our seats because we were trying to escape the pull of the earth’s gravity. It was so hard (5)_______ we could not say anything to each other. Gradually the weight lessened and I was able to talk (6)____ him. “ Why is the spaceship not falling back to the earth? (7)_____ the earth if I fall from a tree I will fall to ground.” I asked. “ We are too far from the earth now (8)______(feel) its pull “ he explained “ so we feel as if there is no gravity at all. When we get closer to the moon we shall feel its gravity (9)______(pull) us but it will not be as strong (10)_____ pull as the earth’s. “ I cheered up immediately and floated weightlessly around in our spaceship cabin (11)__________(watch) the earth become smaller and the moon larger.
When we got there I wanted to explore immediately. “ Come on” I said. “ If you are right my weight will be less (12)_____ on the earth because the moon is smaller and I will be able to move (13)_________(free). I might even grow taller if I stay here long enough. I shall certainly weigh less!” I laughed and climbed down the (14)______(step) from the spaceship. But (15)______ I tried to step forward I found I was carried twice as far as on the earth and fell over. “ Oh dear” I cried “ (16)______(walk) does need a bit of practice now that gravity has changed.” After a while I got the hang of (17)_____ and we began to enjoy ourselves.
Leaving the moon’s gravity was not as (18)________(pain) as leaving the earth’s. But returning to the earth was very (19) _________(frighten). We watched (20) ________(amaze) as fire broke out on the outside of the spaceship as the earth’s gravity increased. Again we were pushed back hard into our seats as we came back to land. “ That was very (21)________(exhaust) but very exciting too” I said. “Now I know much more (22)______ gravity! Do you think we could visit some stars next time?” “Of course” he smiled “ which star would you like to go to?”
答案：
1. exactly
2. accepted
3. that
4. to create
5. What
6. and
7. when
8. that
9. whether
10. which
11. that
12. its
13. unlike
14. to
15. presence
16. harmful
17. it
18. extremely
19. with
20. by
21. called
22. for
23. laying
24. laid
25. However
26. disappearance
27. to
28. to feed
29. with
30. their
31. escaping
32. of
33. on
34. be solved
1. to make
2. to
3. that
4. As
5. that
6. to
7. On
8. to feel
9. pulling
10. a
11. watching
12. than
13. more freely
14. steps
15. when
16. walking
17. it
18. painful
19. frightening
20. amazed
21. exhausting
22. about
本文来源：https://www.2haoxitong.net/k/doc/3405a18d3386bceb19e8b8f67c1cfad6185fe94a.html
文档为doc格式