It is not until we know more about the situation ______ we are able to improve it.

A．that

B．what

C．when

D．which

应当先履行债务的合同一方已经掌握确凿的证据证明合同对方的经济状况严重恶化，所以先行终止了债务履行，这种权利是指( )。

A．不安抗辩权
B．同时履行抗辩权
C．先履行抗辩权
D．后履行抗辩权