我国的商业汇票签发必须有真实的贸易背景。
参考答案：对
—We can use QQ to talk with each other online.
—Really? Could you please show me ______ it? [ ]
A. how to do
B. what to do
C. when to do
D. why to do
墙壁式电缆与煤气管线的垂直交叉净距（）m。
A.0.1
B.0.2
C.0.3
D.0.4
