关于while/when和see sb doing的问题

关于while/when和see sb doing的问题
1.Tom was sleeping_____ Mary was dancing to the music.
A.since B.when C.as D.while
这一题为什么用while不用when
2.When I was walking along the road,I saw a bird_____ over my head.A.flies B.flew C.to fly D.flying
这题为什么用flying不用fly