The equation of a circle with origin as centre and passing through the vertices of an equilateral triangle whose median is of length 3a is

A

$$x^2 + y^2 = 9a^2$$

B

$$x^2 + y^2 = 16a^2$$

C

$$x^2 + y^2 = 4a^2$$

D

$$x^2 + y^2 = a^2$$