Suppose a random variable X follows the binomial distribution with parameters n and p,where 0 < p < 1.If P(x=r)/P(x=n-r) is independent of n and r ,then P equals

Question

$\begin{array}{1 1}(A)\;\frac{1}{2}\quad(B)\;\frac{1}{3}\quad(C)\;\frac{1}{5}\quad (D)\;\frac{1}{7}\end{array}$