--- I don't know ____ makes her different from others.---Confide

--- I don't know ____ makes her different from others.---Confidence, I think.
A、how is it that
B、how it is that
C、what is it that
D、what it is that
【正确答案】：D
【题目解析】：考查宾语从句及强调句型。此处know后为宾语从句，宾语从句中有强调句型。句意：---我不知道究竟是什么使他与众不同。---我认为是自信。