GMAT Sentence Correction (SC) | Re: Among lower-paid workers, union members are less likely than

My doubt in question answer about the use of Comma+ verb ing
So far we know comma+verb ing refers to preceding clause or sub of preceding clause.

But here "spending" refers to "Doctors" which is not the subject of preceding clause.
In exam time only depending on that issue I may eliminate the right choice D.
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