JEE Advanced 2016 Paper 1

Two loudspeakers M and N are located 20 m apart and emit sound at frequencies 118 Hz and 121 Hz, respectively. A car is initially at a point P, 1800 m away from the midpoint Q of the line MN and moves towards Q constantly at 60 km/hr along the perpendicular bisector of MN. It crosses Q and eventually reaches a point R, 1800 m away from Q. Let v(t) represent the beat frequency measured by a person sitting in the car at time t. Let vP,vQ and vR be the beat frequencies measured at locations P, Q and R, respectively. The speed of sound in air is 330ms−1.Which of the following statement(s) is(are) true regarding the sound heard by the person ?