Li Lei ______ in the classroom now. But he______ a moment ago. He ______ to the teacher's office. 　　[ ]

A. is, wasn't, has gone 　　　

B. isn't, was, went 　　

C. is, wasn't, has been　　　

D. isn't, was, has gone

债券质押式回购交易涉及(　　)。

A．两个交易主体

B．一个交易主体

C．两次交易契约行为

D．一次交易契约行为