It was an unpleasant subject we for a long time.
A. about which…discussed
B. about which…argued
C. over them…argued
D. which…argued
网友回答
A. about which…discussed
(答案→)B. about which…argued
C. over them…argued
D. which…argued
解析：考查定语从句和动词：第一空填AB项都可以。因为这是定语从句，而且argue是不及物动词，加介词about/over, discuss讨论，不符合句意：这是一个不愉快的话题，我们讨论了很长时间。选B。