Three point charges + q, - 2q and + q are placed on x axis at x = - d, x = 0 and x = + d respectively. The value of electric field at a point P on x axis at x = r (r >> d) is given by $$E = \frac{aQ}{r^n}$$ where $$Q = \frac{2qd^2}{4\pi\varepsilon_0}$$. Then

A

$$a = 3$$, $$n = 3$$

B

$$a = 6$$, $$n = 4$$

C

$$a = 3$$, $$n = 4$$

D

$$a = \frac{3}{2}$$, $$n = 4$$