I'm revising for an exam and looking at a few exercises, one of which starts with
Consider the charged-current interaction between a muon neutrino with one of the valence quarks of the proton.
We did not discuss things like in detail in the course, so I looked at the solution to get a hint. The first sentence in the solution is
At first order, the charged-current process must be $\nu_\mu d\rightarrow \mu^-u$
Why's that? Why can't the neutrino interact with an up-quark?
Where do the flavour changes in both sectors come from?