问题
单项选择题
超声波是振动频率超过人耳听觉范围的声波，是机械波的一种，其频率高于（）。
A、20kHz
B、2MHz
C、30kHz
D、200kHz
答案
参考答案：A
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超声波是振动频率超过人耳听觉范围的声波，是机械波的一种，其频率高于（）。
A、20kHz
B、2MHz
C、30kHz
D、200kHz
参考答案：A
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| In England nobody under the age of eighteen is allowed (允许) to drink in a public bar (公共酒吧). Mr. Brown usually went to a bar near his house, but he 1 took his son, Tom, because he was too 2 . Then, 3 Tom's 18th birthday, Mr. Brown took him to his usual bar 4 the first time. They drank for half an hour, and then Mr. brown 5 , "Now, Tom, I will tell you. You must always be careful not to drink 6 . And how do you know when you had 7 ? Well, I'll tell you. Can you see those two lights 8 the end of the bar? When they seem to become 9 , you had enough and 10 go home." "But Dad," said Tom, "I can see only one light there!" | ||||
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