The possible long-term health risks to airline personnel who

Dear mbsingh,

I'm happy to respond. I see that BeingHan already gave you an intelligent response, and I will add a few thoughts.

The word "since" is subordinate conjunction. See
GMAT Grammar: “On a White Bus” with Subordinate Conjunctions
This means that it usually is followed by a full [noun] + [verb] clause. That structure is 100% correct.

It's true that, like many subordinate conjunctions, the word "since" also functions and can just take a noun as its object--"since summer," "since this morning," "since the end of baseball season." These are also 100% correct.

Of course a noun inside a prepositional phrase is allowed to have a noun modifier acting on it, but this is slippery slope. If we have [noun] + [participle] inside a prepositional phrase, we are on shaky ground. If the participle truly is just a modifier, just providing decorative detail, then it could be fine. If the [noun] + [participle] unit, though, contains action and is a kind of proxy for a full [noun] + [verb] unit, then we are trying to cram too much action into a prepositional phrase. That's awkward. If we want to talk about action, we need a full bonafide [noun] + [verb] clause. See the discussion in this blog:
with + [noun] + [participle] on GMAT Sentence Correction

Here's (E):
. . . repeatedly crossing multiple timezones have been a concern since the first jet transports going into service in the 1950s.
I'm going say that "jet transports going into service" sounds like an action--it sounds like a tremendously large-scale activity. That's too much action to put inside a prepositional phrase. That's way (E) sounds awkward to native ears. The full clause in (C) after the word "since" is much better.

Does all this make sense?
Mike
_________________

The possible long-term health risks to airline personnel who repeatedly cross multiple time zones has been a concern since the first jet transports went into service in the 1950s.

A) who repeatedly cross multiple time zones has been a concern since the first jet transports went

Subject- Risks (p) has been( s)- incorrect. So Eliminate A

B) who are repeatedly crossing multiple time zones has been a concern since the first jet transports that went

Who (s) - are (P) incorrect
has been(s) and subject Risks- Plural
Clause- since the first jet transport-- Verb missing ( new clause starting with "that" -takes away verb "went")

So Eliminate B


C) who repeatedly cross multiple time zones have been a concern since the first jet transports went

Correct - Plural "have been" is used for plural Subject"risks".


D) repeatedly crossing multiple time zones has been a concern since the first jet transports went

Subject- Risks (p) has been( s)- incorrect. So Eliminate D

E) repeatedly crossing multiple timezones have been a concern since the first jet transports going

clause -since the first jet transport- verb missing "going" is participle we need verb for clause. So Eliminate E

Ans is choice C

Can someone explain how could we equate plural with singular - "risks ... have been a concern"