grammatical number - Referring to a plural noun as singular?

I came across this sentence online:

C extensions are a big part of the Ruby ecosystem.

Now, the word "C extensions" is the subject of the sentence and it's in plural form. The writer wants to say that the ability to write extensions in C (a popular programming language!) is a major part of programming practices inside Ruby (another programming language) environment. With this meaning in mind, is it correct to use the plural auxiliary 'are' to refer to a subject, although in plural form, functions as one unit or as a whole?!

If I were to write the same sentence, I would probably say:

C extensions is a big part of the Ruby ecosystem.

correct?

Answer

Writing are is correct. You use are because the world extensions is plural. If it wasn't (extension) you would have used is.

It doesn't matter if you are referring to every single extensions or to all of them together in this case.

You must use are, that's the rule :)