If the roots of the equation $a x^{2}+b x+c=0$ are equal then $\mathrm{c}=$ ?(a) $\frac{-b}{2 a}$(b) $\frac{b}{2 a}$(c) $\frac{-b^{2}}{4 a}$(d) $\frac{b^{2}}{4 a}$