5. Fill three non-zero real numbers $a, b, c$ into the three boxes of $\square x^{2}+\square x+\square=0$ in any order, and the roots of the resulting equation are all rational numbers. Such real numbers $a, b, c$:
(A) do not exist.
(B) exist uniquely.
(C) exist in two pairs.
(D) exist, more than two pairs.
$\mathrm{D}$