Let G be the centroid of `Delta` ABC , If `vec(AB) = vec a , vec(AC) = vec b,` then the `vec(AG),` in terms of `vec a and vec b, ` is
A. `(2)/(3) ( vec a + vec b)`
B. `(1)/(6) ( vec a + vec b)`
C. `(1)/(3) ( vec a + vec b)`
D. `(1)/(2) ( vec a + vec b)`