If (3, -6), (5, 7) and (13, 4) are the mid point of the sides BC, CA and AB of triangle ABC and if the centroid of the triangle is (a, b), then a + b =

A

$$\frac{39}{3}$$

B

$$\frac{26}{3}$$

C

$$\frac{20}{3}$$

D

$$\frac{8}{3}$$