$pK_{a1}, pK_{a2}$ and $pK_{a3}$ of $H_3PO_4$ are respectively x, y and z. $pH$ of 0.1 M $Na_2HPO_4$ solution is

Question

(a) 1

(b) $\frac{1}{2} (x+y)$

(c) $\frac{1}{2} (y+z)$

(d) $\frac{1}{2} (x+y+z)$