keep doing sth.选择ing形式During extremely hot weather,elephants require both mud and water to keep their skin cool__they have no sweat glands.A.and.B.so.C.because of. D.due to the fact that整道题是这样的。请问选哪个呢？DThe donor hoped that the little__he had done would help the people in the flooded area.A.which.B.that.C.what.D.whereC