5. Given the equation $(2 a+b) x-1=0$ has no solution for $x$. Then the value of $a b$ is ( ).
(A) negative
(B) positive
(C) non-negative
(D) non-positive
5.D.

From the problem, we know that $2a + b = 0$.
Therefore, we have $a = b = 0$ or $a$ and $b$ have opposite signs.