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

胶拼时，涂胶应自上而下涂刷均匀，厚度不宜过厚，以（）为宜，涂胶应尽快完成，涂刷后立即清除预应力孔道口浮浆。胶拼后应采取措施防雨和避免阳光直射，进行覆盖养护。