He suddenly fell ill, prevented him from attending his friend’s wedding.
A. why
B. what
C. that
D. which
网友回答
A. why
B. what
C. that
(答案→)D. which
解析：句意:他突然生病了，这阻止他不能参加朋友的婚礼。这是非限制性定语从句，that不能引导；what不能引导定语从句；why不能在定语从句中作主语，这里先行词是逗号前的句子，which在从句中做主语，故选D。