GMAT Sentence Correction (SC) | Re: I am confused with the use of one of . For ex : Although

The million dollar rule:
One of the X that/who Y--->
ONE OF THE "NOUN"(WILL ALWAYS BE PLURAL) + THAT/WHO+PLURAL VERB.
ex-This is one of the questions that arecorrect.
This rule is for objects, right?

ONE OF THE "NOUN"(WILL ALWAYS BE PLURAL) + SINGULAR VERB
ex-One of the chairs isbroken.
This one is valid only when the phrase is the subject of the sent, right?

Now just as all other rules have exceptions, the above rule alsodoes.
Exception 1 -A (COLLECTIVE NOUN ex-group)