Collective nouns and subject – verb agreement

Which is correct?
As the prevalence of VoIP increases, so ____ the number of attempts to exploit it.
a. do
b. does

Explanation:
The subject in this clause is number. Number is a collective noun. Here’s a handy hint: If number is preceded by a, it’s treated as plural and if it’s preceded by the, it’s treated as singular.

It’s preceded by the here so we need does for agreement: The number does increase. (You wouldn’t say “The number do increase,” right?)

See more in this dictionary.com entry.

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