Do I call this river bet

[Jacob]

20-40 full game. I have AdKd and raise in MP. All fold except button and BB. Button is a calling station who would go to the river with even 4th best pair or couple of over cards. BB is a good solid tricky player.

Flop - 5d6s2c. I bet both call.
Turn - 7d. I bet both call.
River - 5s. BB bets.

Is it a simple call?

There is $350 in the pot. I don't see how the river card helped the BB. I had both Ad and Kd so he coudnt be bluffing a missed flush draw. If he had any pair he would have just check called. Maybe A5. But then he probably would have tried check raise.

Thanks for your comments.