there is no rule to choose, you can choose if you really like the answer
One with the highest rating is picked as BA
No, but EP will add up the votes and choose it for you, sometimes, if there's a clear majority.
...no rule...I like to choose best answer since I asked the question...otherwise the answer that gets the most votes gets BA...and sometimes people don't read all the answers...& for me, since I was asking the question I feel a sense of responsibility...but, a few have slipped by & the most votes won. BTW, I tend to ask many questions, & it is diligent work to keep up on the answers...