单选题：The girl had hardly rung the bell ___ the door was opened su..

We must take measures to stop various kinds of pollution ______ the environment gets worse. [ ]A. unlessB. untilC. beforeD. when

— Can you tell me how to learn English well? — Do more speaking, I think, ______ you will not be good at spoken English. [ ]A. then B. andC. or D. until

Don't promise anything ________ you are one hundred percent sure.[ ]A．whetherB．afterC．howD．unless

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