In a certain bathtub, both the cold-water and the hot-water fixtures leak. The cold-water leak alone would fill an empty bucket in c hours, and the hot-water leak alone would fill the same bucket in h hours, where c < h. If both fixtures began to leak at the same time into the empty bucket at their respective constant rates and consequently it took t hours to fill the bucket, which of the following must be true?
I. 0<t<h
II. c<t<h
III. c/2<t<h/2
(A) I only
(B) II only
(C) III only
(D) I and II
(E) I and III
答案是E
請問為什麼III對?