grammar

liul781

Realtive 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
   

answer:  b


why? thanks a lot

Toefl

Realtive humidity is the amount of water vapor(THAT) the air contains at a certain temperature ..... with the amount it could hold at that temperature
1.从用语态看，humidity被比较，
2.从结构看，定从省略了THAT， 在从句中作宾语，这样就使2种状态下的amount比较