Ok, this is driving me crazy. But here's what I came up with. Probably wrong......
So C is out and 1 in 3 changes the odds to 1 in 2 for A and B or 1/2 = 50%
If A changes cells then he has a 1 in 4 chance ( 1/2 for event of change and 1/2 without change.) Therefore if we take both of those probabilities his overall chance is 1/4 if he changes cells or 25%.
Stay put, or hide in tunnel.......
I am on the edge of my seat!