I tried to call you____ I heard from him,but you were not in

I tried to call you____ I heard from him,but you were not in.
a.since b.while c.until d.as soon as
为什么答案选D
Work hard,_____you may catch up with your classmates soon.
a.or b.but c.and d.yet
为什么选C
I was drawing a horse_____the teacher came in.
a.while b.as c.when d.the moment
为什么选C