单选题：Don't promise anything ________ you are one hundred percent..

She had just finished her homework _____hermother asked her to practise playing the piano yesterday. [ ]A. whenB. whileC. afterD. since

I'd like to study law at university _____ my cousin prefers geography. [ ]A. though B. as C. while D. for

One reason for Mary's preference for this senior high school is _____ she can have access to outstandingteachers and plentiful learning resources. [ ]A. whyB. howC. whatD. that

They hurried to the train station, _____ it was too late. They had to wait another ten minutes. [ ]A. but B. soC. and D. or

We must get up early tomorrow,_______we'll miss the first bus to the Great Wall.[ ]A. so B. or C. butD. however