If $\alpha$ and $\beta$ be the roots of the quadratic equation $x^2-5x+k=0$,then the value of $k$,such that $\alpha-\beta=1$ is _________

Question

$(A) \;k=\large\frac{1}{6}\\(B)\;k=-\large\frac{1}{6}\\(C)\;k=6\\(D)\;k=-6 $