Question

【题目】After he graduated, Qian Xuesen became a researcher who studied rockets and missile (theory/ theories).

Answer 1

【答案】theories
【解析】句意：毕业后，钱学森成为研究火箭和导弹理论的研究者。theory理论。由and可知前后是并列关系，根据rockets可知此处用复数，所以答案是theories。