51、根據(jù)以下內(nèi)容回答51-70題：

　　A tid.al wave is a very large and very destructive wall of water that rushes in from the ocean toward the shore.Many scientists call these huge waves tsunami(海嘯).In Japanese tsunami means“storm wave”0r“tide”.But do you know that tidal waves are not caused bv storms and that they are not true tides at all?A true tide is the regular rise and fall of ocean waters，at def-inite times each day.But a tidal wave comes rushing in suddenly and unexpectedly.A tidal wave is caused by an underwater earthquake.Scientists call the underwater earthquake a seaquake.The word“seaquake”is made up of two words.the word’“sea”which means“ocean”and the word“quake”.To quake means“to shake”or“to tremble”.When a seaquake takes place at the bottom of the ocean，the ocean floor shakes and trembles，and sometimes the ocean floor shifts.It is this shifting that produces the tidal wave.The tidal wave begins to move across the sea at great speed.

　　Tidal waves have taken many human lives in the past.Today scientists can predict when a tidal wave will hit land.They use a seismograph to do this.A seismograph is an instrument that records the strength，the direction，and the length of time of an earthquake or seaquake.It is not possible to hold back a tidal wave，but it is possible to warn people that a tidal wave is coming.This warning can save many lives.

　　According to the article，what causes a tidal wave?

　　A.The rise and fall of ocean water at regular times.

　　B.Seaquakes.

　　C.Storm.

　　D.The great speed of the ocean waves.

　　52、What do we know about tsunami?

　　A.It is a scientific term for seaquakes.

　　B.It iS a Japanese name for true tides.

　　C.It is an African word for hurricanes.

　　D.It is a name of an unknown American scientist.