-Your colleague`s spoken English is beautiful,Lily.-It`s no wonder.She_in the USA for a yearas a visiting professor.A.worked B.has worked C.had worked D.has been working选择A 为什么不选B或C

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-Your colleague`s spoken English is beautiful,Lily.-It`s no wonder.She_in the USA for a year
as a visiting professor.
A.worked B.has worked C.had worked D.has been working
选择A 为什么不选B或C

A表示她曾做过一年的导游,与现在做什么无关
B、D表示她到现在为止已经做了一年的导游,现在还在做;但第一句说明她已经不做导游了(是我的同事),所以不对
C过去完成时要和过去时比较,这里没有过去的时间点去比较,语法不通

for a year是过去时
.has worked C.had worked 是过去做的对现在产生影响,显然他住了一年没产生影响