If the first,second and last term of an A.P. are $a,b$ and $2a$ respectively,then its sum is ______

Question

$\begin{array}{1 1}(A) \; \large\frac{ab}{2(b-a)}\\(B)\;\large\frac{ab}{b-a} \\(C)\;\large\frac{3ab}{2(b-a)} \\(D)\;\text{None of these}\end{array}$