He was so deep in thought，thinking of the problem ________ he knocked into the tree without noticing it.
A．when B．while
C．because D．that
________ all of them are strong candidates，only one will be chosen for the post.
(2011·陕西，19)
A．Since B．While
C．If D．As
Please call my secretary to arrange a meeting this afternoon，or________it is convenient to you. (2011·江西，29)
A．whenever B．however
C．whichever D．wherever
Jack wasn’t saying anything，but the teacher smiled at him________he had done something very clever. (2011·湖南，33)
A．as if B．in case
C．while D．though
At school，some students are active ________ some are shy，yet they can be good friends with one another. (2012·四川，4)
A．while B．although
C．so D．as
I had hardly got to the office________my wife phoned me to go back home at once.
(2012·大纲全国Ⅱ，11)
A．when B．than
C．until D．after
