Two buffers (X) and (Y) of pH 4.0 and 6.0 respectively, are prepared from acid HA and the salt NaA. Both the buffers are 0.50 M in HA. When equal volumes of these two buffers are mixed, the pH of the solution obtained will be (Ka = 1 x 10-5)
(a) 7.8
(b) 6.4
(c) 5.7
(d) 3.9