After visiting Harbin, most of the foreign friends said they would never forget the time ________ they had spent.

A．that

B．what

C．when

D．at which

在工程项目合同管理的基本原则中，合同双方的权利、义务对等体现的是（）。

A.平等自愿原则

B.诚实信用原则

C.公平原则

D.等价有偿原则