Nooneknowsexactly（71）___theearthbegan．Accordingtoawidelyacceptedtheory，theearthbeganwitha"BigBang"，（72）___explodedloudlyandproducedwatervaporandmanygases，（73）___（make）theearth'satmosphere

　　No one knows exactly（71）___the earth began．According to a widely accepted theory，the earth began with a"Big Bang"，（72）___exploded loudly and produced water vapor and many gases，（73）___ （make） the earth's atmosphere．As the earth cooled down，water appeared and stayed（74）___ the surface．It was the presence of water（75）___ allowed the earth to dissolve harmful gases and acids into the oceans and seas，which made（76）___ possible for life to develop．First，the appearance of small plants encouraged the development of early shellfish and all sorts of fish．Next，green plants began to grow on land．（77）___were followed by land animals．So reptiles appeared for the first time．Later，dinosaurs developed．（78）___，they suddenly disappeared．After that，mammals came into being．Finally，humans appeared and spread all over the earth．However，they are putting too much carbon dioxide into the atmosphere，making the earth too hot （79）___ （live） on．So，whether life will continue on the earth will depend on（80）___this problem can be solved．