his return from America, he got down to his research.A. WhileB. InC. AsD. On

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A. While
B. In
C. As
(答案→)D. On
解析：考查固定句型结构。on+n/v-ing表示”一....就”。句意：他一从美国回来，就着手他的研究。故选D。As作介词时意思是“作为”，while不能做介词。