Question

Is ________ Qinghai--Tibet Railway plays an important role in China's railway history the topic that you're coming to?

1. A.
   what
2. B.
   it
3. C.
   that
4. D.
   this

Answer 1

C
句意：青藏铁路在中国历史上起着重大的作用是你们要谈的话题吗？that 引导主语从句；what引导主语从句时，从句要缺成分；其他词不能引导语从句。