(2013·高考北京卷)Experts believepeople can waste less food by shopping only when it is necessar

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A. why
B. where
C. that
D. what
解析：选C。题干的意思是：专家们认为必要的时候才购物可以减少食物的浪费。分析结构可知，此处是experts believe...作主句，空格处引导宾语从句，从句不缺少成分，故用that。why在句中作原因状语；where作地点状语；what作主语、宾语或表语。故正确答案为C项。分析清楚句子结构是解题关键。