smc

Notice that $b$ and $d$ are irrelevant to the problem. Also notice that the negative sign is irrelevant since $c$ can be any real number. So the question is equivalent to asking what values $a$ can take on given the inequality $a<c$. The answer, is of course, that $a$ could be anything, or $\boxed{a\text{can be positive, negative, or zero}}$