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

董事、经理违反《中华人民共和国公司法》第六十条的规定，以公司资产为本公司的股东或者其他个人债务提供担保的，担保合同是否有效？