[单项选择]全口义齿人工牙折断、脱落的原因不包括 （）
A. 塑料人工牙盖嵴部未磨粗糙面
B. 人工牙材质有缺陷
C. 瓷牙连接塑料装置不良
D. 义齿下沉
E. 塑料人工牙盖嵴部被污染

In July 1994 Jupiter, the largest planet in our solar system, was struck by 21 pieces of a comet (彗星). When the fragments (碎片) landed in the southern part of the giant planet, the explosions were watched by scientists here on earth. But what if our own planet was hit by a comet
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 fragments will probably hit the southern part of the Earth. The news has caused panic.
On 17 July, a fragment four kilometers wide enters the Earth’s atmosphere with a huge explosion. About half of the fragment is destroyed. But the major part survives and hits the South Atlantic at 200 times the speed of sound. The sea boils and an enormous wave is created and spreads. The wall of water rushes towards southern Africa at 800 kilometres an hour. Cities on the African coast are totally destroyed and millions of people are drowned. The wave moves into the Indian Oce
A. The whole world becomes extremely cold.
B. All the coastal cities in Africa are destroyed.
C. The whole mankind becomes extinct.
D. The visit of the comet results in wars.