You should never base anything on a theory that impractical in practice.A. is provedB. pr

问题补充：

You should never base anything on a theory that impractical in practice.

A. is proved

B. prove

C. proves

D. proved

答案：

A. is proved

B. prove

(答案→)C. proves

D. proved

解析：考查时态和语态。句意：你不应当基于任何一个不切实际的理论。prove作证明是;结果是;事实说明解时,用作连系动词，没有被动语态。结合语义可知定语从句描述的是客观存存在的事实，故用一般现在时态。逻辑主语a theory是单数，故谓语动词用单数，选C。