I would probably say "A"
Hope this helps.
it could be with or to it means the same thing most people use to
A, The river is constantly changing. I did this last year so I know what the answer is. Fickle means changing frequently or in my term, stubborn. In his point of view, the river has a mind of its own and changes often. Which he described as "fickle." It may be hard for Twain to pilot a steamboat, if the river is constantly changing.
