A grammar question on "It was not until..."

I am not sure about the correct grammar of this phrase. For example:

It was not until 2010…

A.did the family reunite
B.that the family reunited

I instinctually answered A when I saw this question. “It was not until 2010 did the family reunite.”

However, I was suspicious about my answer after seeing a number of people answering B, thinking twice again, B could be right too, that is why I am posting this question here, as I am not sure about it.

Which is the grammatically correct answer? and why? Thanks.

can’t explain by grammatical theory but I have few examples to show.

B. because I would associate “It was not until” with “that”

It was not until I saw him that I realized, he is good-looking.
It was not until midnight that I finally decided to go home.

B is the only correct answer. Let’s have a look for cleft sentences. A also have some relation with this structure. It’s the inversion structure. Not until 2010 did…