— Did you attend the meeting?

— I would have known the news if I       it.

A．had attended

B．attended

C．should attend

D．would attend

对关系S和R进行集合运算，产生的元组属于S中的元组，但不属于R中的元组，这种集合运算称为（ ）。

A．并运算

B．交运算

C．差运算

D．积运算