题目比较多,分不多,见谅了.1.There are five pairs ___,but I'm at a loss w

题目比较多,分不多,见谅了.
1.There are five pairs ___,but I'm at a loss which to buy.
A.to be chosen B.to choose from C.to choose D.for choosing
2.All flights ____because of the snowstorm,many passengers could do nothing but take the train.
A.had been canceled B.have been canceled C.having been canceled D.being canceled.(不是后面的一句过去式吗,为什么不用had?）
3.He was busy writing a story ,only ___once in a while to smoke a cigcrette.
A.to stop B.stopping C.to have stopped D.having stopped.(不是有only to do 为什么不能用?）