By improving reading skills you can read faster and understand more of you once thought i

问题补充：

By improving reading skills, you can read faster and understand more of you once thought impossible to understand.

A. that

B. whether

C. which

D. what

答案：

A. that

B. whether

C. which

(答案→)D. what

解析：通过提高阅读技巧，你会读得更快而且更多理解你曾经认为不可能理解的东西。Of后是宾语从句缺少宾语，think sth impossible.