Question

I just don't understand_____________ that prevents so many Americans from
being as happy as one might expect.

A．why it does	B．what it does
C．what it is	D．why it is

Answer 1

C

本题是宾语从句中的强调句使用。可以将原句恢复：“It is what that prevents  so many Americans…”但是英语中疑问代词或副词需放句首。