If $a,b,c$ are in arithmetic progression , then $\large\frac{1}{\sqrt a +\sqrt b}, \frac{1}{\sqrt a +\sqrt c},\frac{1}{\sqrt b +\sqrt c}$ are in

Question

$\begin{array}{1 1} A.P \\ G.P \\ H.P \\ none\;of\;these \end{array}$