How does grammarly.com make revenue?

I watched youtube video and an ad came out, called Grammarly.

https://www.youtube.com/watch?v=VueSBzrIh1o

As it said, it provides grammar auto-fix for people. And it said it's better than MS office.

So how do you think they make revenue? I saw it's a free Chrome app, actually, no charge...

I am wondering how this startup survives.

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