This machine, for many years, is still working perfectly.
A. after which I have looked
B. which I have looked after
C. that I have looked after
D. I have looked after
网友回答
A. after which I have looked
(答案→)B. which I have looked after
C. that I have looked after
D. I have looked after
解析：which I have looked after 构成一个非限制性定语从句.