AHSME 1993 Problem 10

Let $r$ be the number that results when both the base and the exponent of $a^b$ are tripled, where $a,b>0$. If $r$ equals the product of $a^b$ and $x^b$ where $x>0$, then $x=$ $\text{(A) } 3 \text{(B) } 3a^2 \text{(C) } 27a^2 \text{(D) } 2a^{3b} \text{(E) } 3a^{2b}$