[Solution] . Since the sum of the four roots of the equation is 0 (the opposite number of the coefficient of $x^{3}$), the fourth root of the equation is -6.

From $(x-1)(x-2)(x-3)(x+6)=x^{4}-25 x^{2}+60 x-36=0$, we know
$$
a+c=-25-36=-61 \text {. }
$$

Therefore, the answer is $(D)$.
$2 \cdot 109$ If $x^{4}+a x^{2}+b x+c=0$ has roots $1, 2, 3$, and another one, then $a+c$ equals
(A) 35.
(B) 24.
(C) -12.
(D) -61.
(E) -63.
(17th American High School Mathematics Examination, 1966)