单项选择题

Research is under way to （）

A．discover what causes the sonic boom
B．find a means of eliminating the sonic boom
C．produce the sonic boom at low altitudes
D．reduce the sonic boom to tolerable levels

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单项选择题

A sonic boom is like every sound in that it is （）

A．explosive
B．caused by compressed tar
C．a pressure disturbance
D．shock wave

单项选择题

The article does not say whether the scientists' experiments with dogs have （）

A．benefited animals other than dogs
B．served man
C．helped other dogs
D．contributed to medical knowledge