单选题17.______ my return, I learned that Professor Smith had been at the museum and would not be back for several hours.
【正确答案】
C
【答案解析】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。