The year is 2094．It has been announced that a comet (彗星) is heading towards the Earth．Most of it will miss our planet , but two pieces will probably hit the southern half of the Earth．
On 17 July , a piece four kilometers wide enters the Earth’s atmosphere (大气层) with a massive explosion．About half of the piece is destroyed , but the remaining part hits the South Atlantic at 200 times the speed of sound．The sea boils and a huge hole is made in the sea bed．Huge waves are created and spread outwards from the hole．The wall of water , a kilometer high , rushes towards southern Africa at 800 kilometers an hour．Cities on the African coast are totally destroyed and millions of people re drowned．
Before the waves reach South America , the second piece of the comet lands in Argentina．Earthquakes and volcanoes are set off in the Andes Mountains．The shock waves move north into California and all around the Pacific Ocean．The cities of Los Angeles , San Francisco and Tokyo are completely destroyed by earthquakes．Millions of people in the southern half of the earth are already dead, but the north won’t escape for long．Because of the explosions , the sun is hidden by clouds of dust , temperatures around the world fall to almost zero．Crops are ruined．The sun won’t be seen again for many years．Wars break out as countries fight for food．A year later , no more than 10 million people remain alive．
Could it really happen ? In fact , it has already happened more than once in the history of the Earth．The dinosaurs were on the Earth for over 160 million years．Then 65 million years ago they suddenly disappeared．Many scientists believe that the Earth was hit by a piece of object in space．The dinosaurs couldn’t live through the cold climate that followed and they died out．Will we meet the same and ?
72．What is mainly described in the passage ?
A．A historic discovery．        B．An event of imagination
C．A research on space．      D．A scientific adventure．
73．When the first piece hits the South Atlantic , it causes         ．
A．an earthquake                            B．damages to cities
C．an Earth explosion   D．huge waves
74．Why can’t the northern half of the earth escape for long ?
A．Because the land is covered with water
B．Because the light and heat from the sun can not reach the earth．
C．Because people there can not live at the temperature of zero．
D．Because wars break out among countries．
75．By giving the example of dinosaurs , the author tries to prove       ．
A．animals could not live in the cold climate
B．what happened 65 million years ago was an invented story
C．the human beings will die out in 2094
D．the Earth could be hit by other objects in space