we have common topics

"B" is a little awkward, but not that bad. Studying languages is a single, unitary idea--so you might say "We have a common topic of conversation." It is more likely, though, that a native speaker would say: "We have that in common."

In a sociological perspective, I think these kinds of phenomena refer to Primary and Secondary relationships, of which the primary, in a nutshell, is like you and your relationship with your loved ones, while the latter pertains in contrary of the primary which i think is dependent on a common goal or interest.

I think if you want it to become natural, then you have to build relationship with each other, continue to experience upon interaction with one another.

Thank you~

We have some things in common

Me too! So do I!

Thank you~~