题目: Exchange of answer-backs acts a confirmation that the messages()at the destination.A.have receivedB.have been receivedC.are receivedD.are being received 答案: 参考答案:B
题型:选择题 有三个力,一个力是12N,一个力是6N,一个力是7N,则关于这三个力的合力,下列说法正确的是( )A.合力的最小值为1NB.合力的最小值为0NC.合力不可能为20ND.合力可能为30N 查看答案