Mr. Smith, who

Mr. Smith, who_______ as a teacher in our school for about 20 years, is now our mayor.

has worked

worked

works

had worked

答案及解析：本题考查时态B选项，句意为“史密斯先生在我们学校做了20年老师，他现在是市长”。根据后面的is now our mayor可知，过去20年是老师，现在不是了，强调过去的事实，所以用一般过去时。综上，B选项正确。A选项，现在完成时，与题干时态不符，故排除。C选项，一般现在时，与题干时态不符，故排除。D选项，过去完成时，与题干时态不符，故排除。故正确答案为B项。