问题补充：
I when the UFO landed.
A. am watching TV
B. was watching TV
C. have watched TV
D. watched TV
答案：
A. am watching TV
(答案→)B. was watching TV
C. have watched TV
D. watched TV
解析：本题的从句是过去时，故主句应该是过去时的一种，本题的含义是UFO着落的当时我做的事情应该是正在发生，故用过去进行时，结构为was或were+doing，故选B。
