单选题 (Transforming) raw materials (into) useful products (are) called (manufacturing).
A. Transforming B. into C. are D. manufacturing

【正确答案】 C
【答案解析】[解析] 应改为is。非谓语动词作主语时，谓语动词要用单数形式。本句是动名词短语作主语，谓语要用单数is。