joined or joining?

[Vaok]

The two rivers becom a larger one when _____ together.

A. joined B. joining

I choose B, the suggested answer is A.

I don't know why. what's your opinion? thanks!

[Molly]

In general:

If they are joined by a human act or some unusual act of nature, "joined" is used.

If it's a normal natural act of nature, use "joining".

[Vaok]

reasonable! good! thank you!