请教9710语法一道

ytrrew

12.        Relative humidity is the amount of water vapor the air contains at a certain temperature _____ with the amount it could hold at that temperature.
(A)        to compare
(B)        compared
(C)        comparing
(D)        compares

为什么是B呢？

谁和谁的比值应该怎么说？
A compared B   =   A/B   ？？？是吗？

nbnhjf

a compared with B