Let $r>1$ be a constant. Which of the following illustrates how the graphs of $y=r^x$ and $y=(\frac{1}{r})^{x}$ are related?

(a). One is a reflection over the $x$-axis of the other.
(b). One is a reflection over the $y$-axis of the other.
(c). Both (a) and (b).
(d). None of the above.