In first-order resolution, we resolve clauses by finding a most general unifier (mgu). Consider the following two clauses: $C_1 = P(x, f(y)) \lor Q(x, y)$ $C_2 = \neg P(g(z), z) \lor R(z)$ where $x, y, z$ are variables, and $f, g$ are function symbols. Which of the following is a valid resolvent of $C_1$ and $C_2$?