A. B two cars are driving in the same direction on the straight road. A car has been moving at a constant speed of 20 m / s. when B car is 80 m in front of a car, B car's speed is 4 m / s, and it is moving at a constant deceleration with an acceleration of 2 m / S2? （　　） A. 4sB. 4.2sC. 5sD. 20s

A. B two cars are driving in the same direction on the straight road. A car has been moving at a constant speed of 20 m / s. when B car is 80 m in front of a car, B car's speed is 4 m / s, and it is moving at a constant deceleration with an acceleration of 2 m / S2? （　　） A. 4sB. 4.2sC. 5sD. 20s

If vehicle B decelerates uniformly, with initial velocity V0 = 4m / s and acceleration a = - 2m / S2, it takes time t = V − v0a = 0 − 4 − 2S = 2S for vehicle B to decelerate from 4m / s to zero, and the displacement of vehicle B is XB = v02t = 4m. During this period, the displacement of vehicle a is XA = VAT = 40m ＜ 80m + XB, indicating that vehicle a does not catch up with vehicle B when vehicle B stops. Therefore, the time for vehicle a to catch up with vehicle B is ta = XB + 80va = 4.2s