GMAT Sentence Correction (SC) | Re: Some scientists have been critical of the laboratory tests

dear experts, ormikemcgarry

I cross off D because I think that Subject "the amounts of XXX" is singular and that verb should be "exceeds" .

unfortunately, correct verb is plural, I am confused.

another question is that whyamounts of XXX, IMO, amount is modifies uncountable noun, soamount of XXX is singular.the amounts of XXX is first time for me, would you please clarify how should I distinguish plural or singular amongthe amount of XXX, a mount of XXX , and the amounts of XXX,