A good example of this occurred with a group of dogs. They were shut in an area that was being shaken by a series of tiny earthquakes.(Several small quakes often come before or after a large one.) Before each quake a low booming sound was heard. Each boom caused the dogs to bark wildly. Then the dogs began to bark during a silent period. A scientist who was recording quakes looked at his machine. It was acting as though there were a loud noise too. The scientist realized that the dogs had reacted to a booming noise. They also sensed the tiny quakes that followed it. The machine recorded both, though humans felt and heard nothing.

　　In this case there was a machine to monitor what the dogs were sensing. Many times, however, our machines record nothing extraordinary, even though animals know a quake is coming. The animals might be sensing something we measure but do not recognize as a warning.Discovering what animals sense, and learning how they know it is a danger signal, is a job for future scientists.

　　20．Through the passage the writer hopes to explore ________．

　　A．why animals send a danger signal before an earthquake

　　B．how animals know when an earthquake is coming

　　C．why animals not humans have a good sense of danger

　　D．how much animals know about an earthquake

　　21．During an earthquake in China in 1975 ________．

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