If $a>0$, then $\sqrt {a+\sqrt {a+\sqrt{a+............}}}$=?

Question

$\begin{array}{1 1}(A)\large\frac{\sqrt{4a-1}}{2} \\ (B) \large\frac{1+\sqrt{4a+1}}{2} \\ (C) \large\frac{1-\sqrt{4a-1}}{2} \\(D) \large\frac{1\pm\sqrt{4a+1}}{2} \end{array}$