Moviegoers know that many specia

Moviegoers know that many special effects are created by computers. （ ）they often don't know is that these scenes still require a lot of work.

That

Whom

What 

​How

本题考查主语从句引导词。

is是系动词，所以其前面的内容应该是主语从句作主语。主语从句中缺少know的宾语，而且宾语指物，所以填what。

句意：影迷们知道许多特效是由电脑制作的。他们通常不知道的是，这些场景仍然需要做大量的工作。

所以本题选C。