What's the integral of a constant?
Remember that the fundamental theorem of calculus says (essentially)
if $F$ is an antiderivative of $f$ (i.e. $F' = f$), then $\int f = F + C$.
So, answer your own questions:
1) What kinds of functions have constants as their derivatives (i.e. given $f = $constant, what could $F$ be?)
2) What kinds of functions have $0$ as their derivatives (i.e. given $f = 0$, what could $F$ be?)
The integral of a constant $C$ with respect to $x$ is $Cx + A$, $A$ constant. Applying this rule to the constant function $y(x) = 0$, $\int {0}dx = 0+A = A$.