A tower stands at the centre of a circular park. A and B are two points on the boundary of the park such that AB (=a) subtends an angle of $60^{\circ}$ at the foot of the tower and the angle of elevation of the top of the tower from A or B is $30^{\circ}$. The height of the tower is :

Question

( A ) $a \sqrt{3}$
( B ) $a/\sqrt{3}$
( C ) $2a/\sqrt{3}$
( D ) $2a \sqrt{3}$