Sentence Correction

Hello.
Question regarding subject/verb agreement. Below is a sentence from one of the MGMAT problem sets that ask you to identify the subject and correct the verb if need be.

I was so thirsty, that either of the two drinks were fine with me.


I thought the subject of the sentence is "two drinks" but the MGMAT guide says the subject is "either." Can someone please explain to me with "either" is the subject of the sentence


Thanks,
-Steve

Its not just either, Subject should be either drink. And hence should be singular.

The correct sentence formation, according to MGMAT is:


I was so thirsty that either of the two drinks was fine with me

From your example, it is easy to spot the singular "either drink" subject, but when the answer has "two drinks" in it, is hard for me to spot either as the subject.