$2 \cdot 131$ The number of roots of the equation $\sqrt{x^{2}+12 x+36}+x+6=0$ is
(A) 0.
(B) 2.
(C) 4.
(D) Infinitely many.
(3rd "Five Sheep Cup" Junior High School Mathematics Competition, 1991)
［Solution］The original equation can be transformed into
$|x+6|+x+6=0$, or $|x+6|=-(x+6)$, $\therefore x+6 \leqslant 0$, which gives $x \leqslant-6$, meaning the equation has infinitely many solutions. Therefore, the correct choice is $(D)$.