A palm tree stands on the bank of a river. A post is fixed in the earth on the other bank just opposite to the palm tree. On moving `7\sqrt{3}` metres from the post along the bank, it is found that the tree makes an angle of `60^\circ` at that point with respect to this bank. The width of the river is -
(a) 20 m., (b) 21 m., (c) 22 m., (d) 24 m.