The harder we study the more questions we think of .A. askedB. asking　C. to askD. ask

问题补充：

The harder we study, the more questions we think of .

A. asked

B. asking

C. to ask

D. ask

答案：

A. asked

B. asking

(答案→)C. to ask

D. ask

解析：句意：我们学习越努力，就会想到更多的问题可以问。这里we think of是定语从句修饰questions，横线部分也是非谓语做后置定语修饰questions，因为是将要问的问题，所以用不定式表示将来的含义，选C。