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.（ ）
句意：常看电影的人知道许多特效都是通过电脑制作出来的。但他们往往不知道的是制作这些场景还需要投入大量的工作。分析句子结构可知，空格所在从句在整个句子中充当主语，从句中缺少宾语，且宾语指的是事而非人，故选C。
