--what do you think of my answer to the question?
--Sorry.What is that?___about something else.
空格应该填两个词 was thinking
Sorry.What is that?i was thinking about something else.
对不起,那（你的答案）是什么?我分心了.
was thinking 是过去式,因为第二句里的"我分心了”是说 你说答案的时候我分心了,你说答案的时候已经过去了.
而不用thought 是因为 i thought about something else,突出了 “我想到了其它的事”,而忽略了“你说答案的时候” 我想到了其它的事.所以用thought 没有解释我为什么没有听到你的答案.