大型钢轨探伤车采用的是美国SYS-1900检测系统，该系统发射的超声波由四个轮式探头中发出，每个轮式探头中有4个换能器。
参考答案：错
Excuse me, Miss Wang, a man named Robert from the Bank of Construction ____ all morning.
Could you speak to him now? [ ]
A. phones
B. has phoned
C. has been phoning
D. was phoning
在单质的晶体中，一定不存在[ ]
A．分子间的作用力
B．离子键
C．共价键
D．金属离子与自由电子间的作用
