When I asked them _________ they were afraid, they firmly denied

When I asked them _________ they were afraid, they firmly denied it.
A、what
B、that
C、if
D、why
【正确答案】：C
【题目解析】：if有“是否”的意思。原句的意思是“当我问他们是否害怕了的时候，他们止口否认。”。因此我们选择C. if。