Answer :
The answer is C.
To alternate is to change between different things, so if I were to alternate between two different chairs, I'd be changing between which chair I was using.
To alternate is to change between different things, so if I were to alternate between two different chairs, I'd be changing between which chair I was using.
In this case, used instead of something else is the best option. That would mean that she figured out a path different to the one that she used to use, hence an alternate one.