Binary Exponential Backoff Question. Help pls.

So I've been revising for my Networks exam in a couple of days and I've been arguing with a mate about what would happen in a specific example of Binary Exponential Backoff and can't find a similar example anywhere so wondering if anyone can help me out.

Here's the example:

Lets say A and B are transmitting packets A1, A2... B1, B2... etc.

• A1 and B1 Collide
• A and B pick randomly from {0T, 1T} each
• Lets say A wins, transmits A1
• A2 and B1 Collide
• A has {0T, 1T} and B has {0T, 1T, 2T, 3T}
• Lets say B now wins and transmits B1
• A2 and B2 Collide
• Here's the question: what timings do A and B now have to pick from?

Please let me know if I've made any mistakes so far as well but I think that's right.