Mass deaths of birds were not unheard of, but they normally happened unnoticed at sea or in rural areas away from towns. Many birds are weak and die in winter food is scarce.
A. which
B. what
C. when
D. how
网友回答
A. which
B. what
(答案→)C. when
D. how
解析：考察定语从句。本题定语从句的先行词是winter，后面定语从句中的句中成分很完整，故使用关系副词when引导该定语从句。BD两项不能引导定语从句，which在定语从句中做主语，宾语，表语等。句意：很多鸟很虚弱都死在食物紧缺的冬天。故C正确。