20.No one knows exactly how the earth began.Each(61)religion (religious) and culture has its own ideas about how life began on earth.However,according to a widely accepted theory,the universe began with a"Big Bang".It exploded (62)violently (violent) with fire and rock.They were in time to produce carbon dioxide,nitrogen,water vapour and other gases.Water had also appeared on other planets like Mars (63)but it had disappeared later.(64)Whatmany scientists believe is that the continued presence of water allowed the earth to dissolve (65)harmful (harm) gases and acids into the oceans,which made it possible for life (66)to develop (develop).Many millions of years later,the first extremely small plants (67)multiplied (multiply) and tilled the oceans and seas with oxygen.Later when the plants grew into forests,reptiles appeared for the first time.They produced young generally by (68)laying (lay) eggs.After that,some huge animals,(69)called (call) dinosaurs,developed.However,(70)65million years ago the age of the dinosaurs ended.These disappearance made possible the rise of mammals on die earth.These animals where different from all life forms in the past,because they gave birth (71)toyoung baby animals and produced milk to feed them.