题目内容

One of the boys ________ doing __________ homework in the classroom.
A. is; their      B. are; his      C. is; his
BC解析:
a点是波峰与波峰相遇处，b点是波谷与波谷相遇处，它们的连线上均总是干涉加强的位置，e点是波峰与波谷相遇处，总是振动最弱点，AC错误、B正确；当振源的振动频率同时增加时，波长发生变化，则干涉区域的干涉图样将发生变化，D错误。略
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