Singular Plural?

There is always a reason and here it is ::

Snippet from article published here https://online.wsj.com/public/resources/ ... S.0706.pdf

The stylebook’s entry on collective nouns
advises that with words such as variety, number
and total, a rule of thumb is to use a singular
verb when the article the precedes the noun
and a plural verb when the article a is used.

To understand the context of following information please refer link : contrary-to-popular-belief-video-games-arent-just-for-107348.html#p1378095

This comment was originally posted on above mentioned link, refer question + Adam's comment as well in conjunction.

A variety of bird is very popular (correct)
A variety of birds are in our building (correct)
The variety in languages is rare. (correct)

So there was something more left to learn besides "a number of".

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