A/B testing random, and other questions
When creating an A/B test on an already created email, how do you know who will be receiving the email?

I understand that you can create an email program and then select the audience with whom you want to conduct the A/B test, but that option doesn’t appear to be present when right clicking on an email and then setting up the A/B test. Also, does Marketo randomly choose who will get the Test? Say I choose 10% of those in the Smart List to get the test. Does Marketo randomly choose who will be part of that 10% and who will make up the other 90%? I’m assuming yes, but just want to verify.
Thank you,
Laura