题目内容：
根据以下材料，回答题Passage 3
The year is 2094. It has been announced that a comet 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 theEarth'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 seabed. 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 are 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 earth-quakes.
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 end?
( ) What is mainly described in the passage? A．A historic discovery
B．An event in imagination
C．A research on space
D．A scientific adventure
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