To say something --- one thing , to do it --- another.

问题描述:

To say something --- one thing , to do it --- another.
A is;are
B are ;is
C is;is
D are;are
求解,感谢!

C
都是把不定式看成一个整体,作为主语不懂不定式分句(即to do型的词组,题中就是To say something和To do it 两个)