17・110 Given that the sides of a right triangle are $a, a+d$, and $a+2d$, where $a$ and $d$ are positive numbers, then the ratio of $a$ to $d$ is 
(A) $1: 3$.
(B) $1: 4$.
(C) $2: 1$.
(D) $3: 1$.
(E) $3: 4$.
(10th American High School Mathematics Examination, 1959)