I've got a burning question!

Declining revenues resulting from a decrease in business travel, a source of income without which most commercial airlines could not survive, are going to force many commercial airlines to increase prices and decrease services in the coming months.

(A) a source of income without which most commercial airlines could not survive, are
(B) a source of income without which most commercial airlines could not survive, is
(C) and most commercial airlines use it as a souce of income to survive with, are
(D) which is a source of income which is needed by most commercial airlines who could not survive without it, are
(E) which most commercial airlines use as a source of income without which they are unable to survive, is






Before I reveal the answer. I have something that I would like to ask:

Do you say:
Declining revenues are going to force many commercial airlines...
or
Declining revenues is going to force many commercial airlines...

If you opt for the former, then do you say:
Identifying grammatical errors is a funny thing to do
or
Identifying grammatical errors are a funny thing to do
or
Identifying grammatical errors are funny things to do


Thank you in advance

Btw, the answer is A

Here are some more details about the difference between "declining revenues" and "identifying grammatical errors".

In "declining revenues", the word "declining" is a participle: a verb form that is serving as an adjective. The word which it is modifying, "revenues" is the word that would have been the subject if "declining" were an actual verb ("decline") instead of a participle. "Revenues" is the subject with which the actual verb of the sentence ("are", which comes much later) must agree.

In "identifying grammatical errors", "identifying" is a gerund: a verb form that is serving as a noun. So in this case, "identifying" is itself the subject of a sentence. A gerund is always singular, and so the verb for which it is the subject must be singular.

This difference between a participle and a gerund is famous in linguistics, because Noam Chomsky used it to prove that the grammar of English cannot be expressed using a Markov or "finite state" process. Chomsky pointed out that the sentence "Flying planes can be dangerous" means two different things, depending on whether "flying planes" is interpreted as "planes which fly" (where "flying" is a participle) or "the activity of flying planes" (where "flying" is a gerund). For reasons which I don't quite understand, a Markov process cannot express this kind of difference.
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Grumpy

Kaplan Canada LSAT/GMAT/GRE teacher and tutor