A and B start from the same point and in the same direction at 7 a.m. to walk around a rectangular field 400 m x 300 m. A and B walk at the rate 3 km/hr and 2.5 km/hr respectively. How mar times shall they cross each other if they continue to walk till 12:30 p.m.? a. not even once     b. once        c. twice         d. thrice