Two towns A and B are connected by a regular bus service with a bus leaving in either direction every T minute. A man cycling with a speed of 20km per hour in the direction of A to B notices that a bus goes past him every 18 minutes in the direction of his motion,and every 6 minutes in the opposite direction. The period of bus service is

A-18 minutes        B- 6 minutes

C- 9 minutes         C- 12 minutes