I won’t tell the student the answer to the math problem until he ________ on it for more than an hour.

A．has been working

B．will have worked

C．will have been working

D．had worked

在商业助学贷款中，当借款人和自然人保证人的工作单位及通信方式发生变更时，借款人应提前（）天通知贷款银行。

A.10

B.15

C.20

D.30