设关系R与关系S具有相同的目(或称度)，且相对应的属性的值取自同一个域，则 R-(R-S)等于

A．R∪S

B．R∩S

C．R×S

D．R-S

Several witnesses gave accounts of what they had seen. It took many hours to _______ the court of the driver's guilt.

A．prove

B．convince

C．persuade

D．demonstrate