知识问答
最佳答案:tom have already finished his homework three hours ago.he finished his homework
最佳答案:①产生机械波需要两个条件同时达成:①机械振动②有介质存在.这里只有机械振动而没有介质,所以不一定能产生机械波.②声音是有物体振动产生的,若振源停止振动,则声音停