When is a set of functional dependencies $F$ said to be minimal? a) Every dependency in $F$ has a single attribute for its right-hand side. b) We cannot replace any dependency $X \to A$ in $F$ with other dependency $Y \to A$, where $Y$ is a proper subset of $X$, and the changed set of dependencies is still equivalent to $F$. c) We cannot remove any dependency from $F$ and still have a set of dependencies that is equivalent to $F$. d) Every dependency in $F$ has more attributes for its right-hand side.