信用证的特点之一是独立于贸易合同。( )
参考答案：对
—Do you know that Mrs. Obama came to China with her mother and two daughters?
—Yes. And it is their ______ visit to China.
A．one
B．first
C．the first
D．once
旅客广播系统的音频输入来自于（）。
A.磁带放音机
B.驾驶舱麦克风
C.服务员话筒
D.A+B+C
