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Experts believe________people can waste less food by shopping only when it is necessary.????????????? ????????????? ????????????? ????????????? ????????????? ????????????? ????????????? ????????????? ????????????? ????????????? ????????????? (2013·北京，33)
A．why? B．where
C．that? D．what
(2013·高考北京卷)Experts believe________people can waste less food by shopping only when it is necessary.