question help: English grammar question, - Help.com

Just my opinion here, but the first would state that x is the most important, and that y isn’t. Where the second one would say x and y are equally important.

if you talk about one thing or one person(number a),if you talk about more than one thing or more than one person(number b )

According to me. there are many question in English grammar question,
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The word “and” could be substituted for “as well as,” and the sentence could then be read: “Most important of which are x and y.” The term “as well as” serves as a conjunction and you thus have a plural subject, which demands a plural verb.