⑬it为形式主语，whether从句为真正的主语。

　　⑭in time to do sth./for sth.及时做某事；来得及做某事

　　in time及时；终于

　　⑮carbon dioxide二氧化碳　carbon/'kɑːbən/ n．碳

　　dioxide/daI'ɒksaId/ n．二氧化物

　　⑯nitrogen/'naItrədʒən/ n．氮

　　⑰water vapour 蒸气；水蒸气

　　vapour/'veIpə/ n．水蒸气；蒸气

　　⑱atmosphere/'ætməsfIə/ n．大气层；气氛

　　⑲cool down 凉下来，冷却

　　地球上生命的起源

　　[第1～2段译文]

　　没有人确切知道地球是怎样开始的，因为在很久以前它就形成了。然而，根据一种普遍为人们所接受的理论，宇宙起源于一次大爆炸，这次大爆炸将物质投射到四面八方。然后，原子开始形成并结合成恒星和其他天体。

　　大爆炸后好几十亿年过去了，地球还只是一团尘埃。(随后)它变成什么没人确定，直到45～38亿年前，这团尘埃才慢慢地形成一个固态的球体。地球变得如此激烈动荡，以至于不知道这个固体形状是否会继续存在下去。它(地球)猛烈的爆炸喷出了烈火和岩石，最终产生了二氧化碳、氮气、水蒸气和其他气体，从而形成了地球的大气层。更重要的是，随着地球冷却了下来，地球的表面开始出现了水。

　　Water had also appeared on other planets like Mars but, unlike⑳ the earth, it had disappeared later.It was not immediately obvious that water was to be fundamental\s\up1(21(21) to the development of life\s\up1(22(22).What many scientists believe is that the continued presence of water\s\up1(23(23) allowed the earth to dissolve\s\up1(24(24) harmful\s\up1(25(25) gases and acids\s\up1(26(26) into the oceans and seas.This produced a chain reaction\s\up1(27(27)， which made it possible for life to develop.

Many millions of years later, the first extremely small plants began to appear on the surface of the water.They multiplied\s\up1(28(28) and filled the oceans and seas with oxygen\s\up1(29(29)， which encouraged\s\up1(30(30) the later development of early shellfish\s\up1(31(31) and all sorts of fish.Next, green plants