题目

静止的实验火箭，总质量为M，当它以对地速度v0喷出质量为△m的高温气体后，火箭的速度为（） A . v0 B . ﹣v0 C . v0 D . ﹣v0 答案：B从所给的四个选项中选择最佳答案 The duties of the secretary are to receive visitors, ______ ． [　　] A．opening the mail, she types letters B．to open the mail and typing letters C．to open the mail and type letters D．to open the mail and they type letters