Question

I ________ when the UFO landed.

1. A.
   am watching TV
2. B.
   was watching TV
3. C.
   have watched TV
4. D.
   watched TV

Answer 1

B
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。