求助prep46

46.   (26975-!-item-!-188;#058&003041)

The continental United States receives an average of 30 inches of precipitation a year; transpiration from soil and from plants returns approximately 21 of the 30 inches to the atmosphere, while the balance of 9 inches contributes to the flow of streams and rivers.

(A) transpiration from soil and from plants returns approximately 21 of the 30 inches to the atmosphere, while the balance of 9 inches contributes to the flow

(B) even though transpiration from soil and from plants returns approximately 21 of the 30 inches to the atmosphere, the remainder of 9 inches contribute to the flowing

(C) although transpiration from soil and from plants return approximately 21 of the 30 inches to the atmosphere, the balance of 9 inches contribute to the flowing

(D) with transpiration from soil and from plants to return approximately 21 inches to the atmosphere, the rest of the 9 inches contributes to the flow

(E) as transpiration from soil and from plants return approximately 21 of the 30 inches to the atmosphere, the rest of the 9 inches contribute to the flow

请问一个有关单复数的问题， 为什么说contribute是错的，the remainder/rest of 后面的谓语动词不是由of后面的名词单复数决定的吗？

通常LZ講的規則是成立的，但是並非絕對，本題中21 of 30 inches需要視為一個整體來看，就是21吋的水回到大氣,其他部分則流到河中,邏輯上視為複數是講不通的，因為不可能算一寸一吋的水然後說有21個1吋的水回到大氣中.