by与through都不错

有一读者问： 有这样一道题：

This is one of the means_________the electric energy is conducted from one place to another.

A. by which　B. by that

C. through which　D. through that

答案为A，请问为什么？

此题不够严谨。means（作为手段解）之前通常皆用by，但偶尔也可用through。又，关系代词which之前可用介词，that一般则不可。