Two Radio Antennas Are 10 Km Apart on a North-South

Two radio antennas are 10 km apart on a north-south axis on a seacoast.The antennas broadcast identical AM radio signals,in phase,at a frequency of 4.70 MHz.A steamship,200 km offshore,travels due north at a speed of 15 km/hr and passes east of the antennas.A radio on board the ship is tuned to the broadcast frequency.The reception of the radio signal on the ship is a maximum at a given instant.The time interval until the next occurrence of maximum reception is closest to:

A) 5.1 min
B) 3.8 min
C) 6.4 min
D) 7.7 min
E) 8.9 min