78. Although dozens of New York’s small museums are either devoted to local history or various ethnic groups, there are many one-of-a-kind museums from Manhattan to the Bronx that are open for exploration on summer weekends.

(A) Although dozens of New York’s small museums are either devoted to local history or various ethnic groups, there are
(B) Although dozens of New York’s small museums are devoted to local history or various ethnic groups
(C) Dozens of New York’s small museums are devoted to local history or various ethnic groups, but there are
(D) Dozens of New York’s small museums are devoted to local history or various ethnic groups, and there are also
(E) Devoted to local history or various ethnic groups, dozens of New York’s small museums and also

Why D and not C?

78. Although dozens of New York’s small museums are either devoted to local history or various ethnic groups, there are many one-of-a-kind museums from Manhattan to the Bronx that are open for exploration on summer weekends.

(A) Although dozens of New York’s small museums are either devoted to local history or various ethnic groups, there are
(B) Although dozens of New York’s small museums are devoted to local history or various ethnic groups
(C) Dozens of New York’s small museums are devoted to local history or various ethnic groups, but there are
(D) Dozens of New York’s small museums are devoted to local history or various ethnic groups, and there are also
(E) Devoted to local history or various ethnic groups, dozens of New York’s small museums and also

Why D and not C?

First the sentence requires the first part to subordinate the second part So choice C, D, E are out.

Between choice A and B Choice A incorrectly uses Either X or Y. So the answer is choice B

There's something about this question that just rubs me the wrong way; the relationship between the two parts of the sentence just isn't that clear. Although 1000SC has some value, I hope you've exhausted better resources (like both OG's and our problem banks) before using it.

As for 'why D and not C': Choice C, like choices A and B, contains a transition that indicates some sort of contrast between the two parts of the sentence. (This transition is 'but' in the case of choice C, and 'although' in the case of the first two choices.) If you examine the meaning of the sentence, that contrast just isn't there; the meaning is better served by '...and also' (because the sentence is talking about additional museum options).