Two circles of radii 15 cm and 12 cm intersect each other, and the length of their common chord is 18 cm. What is the distance (in cm) between their centres ?

A

$$12 + 3\sqrt{7}$$

B

$$15 + \sqrt{7}$$

C

$$12 + 2\sqrt{7}$$

D

$$18 + \sqrt{7}$$