单选题 ______ my return, I learned that Professor Smith had been at the Museum and would not be back for several hours.
A. At B. With C. On D. During

【正确答案】 C

【答案解析】[解析] on用来指时间，表示“在……之时”，相当于at the same time of，后跟动名词或表示动作的名词，例如：On his arrival in Qingdao, he set to work in earnest. 一到青岛他就认真地干了起来。而at，with和during则没有这种用法。