the rain falling so frequently ,it becomes more and more difficult to carry on

the rain falling so frequently ,it becomes more and more difficult to carry on
_______ the rain falling so frequently ,it becomes more and more difficult to carry on the rescue work.
A.since B.with C.as D.for\
应该选B.为什么不是A?众所周知的原因“由于雨下的很频繁,营救工作很难进行”