5. If $(a, b)$ and $(c, d)$ are two points on the line $y=m x+k$, then the distance between $(a, b)(c, d)$ expressed in terms of $a, c, m$ is
A. $|a-c| \sqrt{1+m^{2}}$
B. $|a+c| \sqrt{1+m^{2}}$
C. $\frac{|a-c|}{\sqrt{1+m^{2}}}$
D. $|a-c|\left(1+m^{2}\right)$
5. A Hint: $b=m a+k, d=m c+k$, so $b-d=m(a-c), \sqrt{(a-c)^{2}+(b-d)^{2}}=|a-c| \sqrt{1+m^{2}}$