D. $\frac{ax_{0}+ by_{0}+ c}{\sqrt{a^{2}+b^{2}}}$
C. $\left | \frac{ax_{0}+ by_{0}+ c}{a^{2}+b^{2}} \right |$
A. $\frac{ax_{0}+ by_{0}+ c}{a^{2}+b^{2}}$