Thanks to the measures ______ by the local government , the polluted river is becoming clearer and clearer and fish have begun to show up in it.

A．taking

B．taken

C．to take

D．having taken

合同成立以后，尚未履行完毕之前由合同当事人双方依法对原合同的内容进行修改，称为( )。

A．合同变更

B．合同转让

C．合同撤消

D．合同解除