Moviegoers know that any special effects are reeled by computers. _____ they often don't know is that these scenes still require a lot of work.
A、That
B、Whom
C、What
D、How
【正确答案】：C
【题目解析】：they often don't know作后面句子的主语从句，know后面缺少宾语，选项中能作宾语的只有what和whom，根据is that后面的表语从句可知，只能用what。