Countercurrent heat exchange, a circulatory arrangement in which warm

The correct answer is A.

Here is why:

Warmth is an uncountable modifier so you cannot use few this gets you down to A and B. And regions is plural so you need exist which gets you to A.

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I am going with E

A. little warmth or heat mechanisms exist--- this sentence doesnt make sense-- how can we have little heat mechanism ( as there is nothing after or)
B. little warmth or heat mechanisms exists--- (same)
C. few heat mechanisms and little warmth exists ( exists-- is incorrect here)
D. there is little warmth or heat mechanisms available ( same as A)
E. there are few heat mechanisms and little available warmth

Please help me understand why option C is incorrect and why option E is correct.

Further request you to elaborate which noun is the verb 'exists' referring to in option and why is it incorrect. In my understanding exists is a singular verb for a singular noun.

Thank you in advance.

Meaning : Self explanatory

Errors in the original sentence :
1) heat mechanisms is countable. How do you check whether it is countable? Because mechanisms is plural. You can say 1 song, 2 songs and so on but can you say 1 music, 2 musics ? No. Therefore, if you are confused whether it is a countable or uncountable noun, you can check whether it is plural.

Okay, I don't see any other error. So, lets move on to other choices.

Countercurrent heat exchange, a circulatory arrangement in which warm blood leaving the body core passes cooler blood returning from the extremities, allows for heat to pass into regions where little warmth or heat mechanisms exist.

A. little warmth or heat mechanisms exist - As discussed above
B. little warmth or heat mechanisms exists - Same error as above. Moreover, mechanisms is plural. Therefore, exist should be plural. Why? Whenever you see or in the sentence (either...or... ; neither... nor...; X or Y ... stick with the SV Agreement of the later noun
C. few heat mechanisms and little warmth exists Because of the presence of "and" the choice should have been plural
D. there is little warmth or heat mechanisms available Same as A. You need few heat mechanisms to maintain //ism with "little warmth"
E. there are few heat mechanisms and little available warmth SV Agreement - CHECK. //sim with few and little - CHECK

Ans : E

E is correct, simple rule of countable snd uncountable. Only option E does the job perfectly.

These kind of clinical tests are destructive for a gmat student. Even the answer given is wrong. Check out this page from magoosh and you will know why A is the correct answer, and moreover, Option E is ****, doesn't make sense.

https://magoosh.com/gmat/verbal/sentence-correction/subject-verb-agreement-on-gmat-sentence-correction/

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