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


Source GMAT Prep.

isn't transpiration from soil and from plants compound subjects?

I always thought AND makes a subject compund???

nope.

this occurrence of 'and' does not make the subject compound, because it is contained within the prepositional phrases that modify the subject. the subject is still one word (transpiration), and is not in any way plural.

in particular, the subject is not transpiration AND anything else - it's just transpiration. there is more than one descriptive phrase, but those phrases are not part of the subject.

Is OA A?

yep, answer is A

is 'rest of 9 inches' singular as 'balance of 9 inches' ?

yes.

notice that constructions involving numbers and units are, generally, singular - even if they aren't prefaced by such things as 'balance' or 'rest'. for instance,
fifteen dollars is far too much to pay for this item --> correct
fifteen dollars are far too much to pay for this item --> incorrect

same with feet, gallons, inches, ohms, amps, etc. any quantity measured in these sorts of units is almost always singular, since units aren't things that are literally counted.