. _______ 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
【题目解析】：C【句意】我一回来就了解到史密斯教授已经到了博物馆，几个小时不会回来。【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，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。