题目:
With the help of modern technology, ships are able to receive automated broadcast of safety messages().
A.by day or at night
B.almost round the clock
C.somewhat and sometime
D.from sunrise and sunset
答案:
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参考答案:对解析: 超声换能器的原理是通过声能与电能的相互转换产生和接收超声波的。发射换能器是将电能转化成声能,即产生并发射超声波,超声波在混凝土中传播后,被接收换能器接收,并将超声能量转换为电能...