The factory used 65 percent of the raw materials, the rest of which saved for other purpo

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A. is
B. are
C. was
(答案→)D. were
解析：考察主谓一致。Which指代先行词materials，当the rest在句中作主语时，谓语动词的单复数取决于后面的名词，如果是名词是不可数名词，用单数。如果是可数名词复数，谓语动词用复数形式。本句中的materials是可数名词复数形式。