It is only when the disease has progressed to the AIDS a person begins to look sick.A. w

问题补充：

It is only when the disease has progressed to the AIDS a person begins to look sick.

A. which

B. what

C. who

D. that

答案：

A. which

B. what

C. who

(答案→)D. that

解析：句意：只有当疾病发展成艾滋病时，一个人看起来才像是病人。这里是强调句型，其结构是It was+被强调部分+ that+其他，判断强调句型的方法是把It was that去掉整个句子不缺成分，是成立的。这里强调的是时间状语从句，故用that。