单选题 ______ his return from Japan, Prof. Li went directly to his laboratory and started working with his colleagues.
A. Upon B. At C. Within D. Over

【正确答案】 A
【答案解析】[解析] 本句意为“李教授一从日本回国就立刻进入实验室开始与同事们一起工作。”“upon (on) +表示行为的名词”相当于as soon as引导的时间状语从句。