21. John skates on the ground subject to a friction force. For a stationary observer A relative to the ground, he observes that John has an initial velocity +5 m/s in the x- direction and finally stops after traveling a distance of 12.5 m. This phenomenon is observed by the other observer B, who is riding a bus that moves at a constant speed of +2 m/s in the x-direction relative to the stationary ground. Therefore, the observer B sees that John's velocity changes from +3 m/s to -2 m/s in the x-direction. Consider the kinematic equation that, where △x is the distance travelled； which of the following statements is true? (A) From the observer B's point of view, since , B would measure a different deceleration than what A would have measured. (B) The kinematic equation only works in A's reference frame. (C) The kinematic equation only works in B's reference frame. (D) The inconsistency of the kinematic equation can be amended by the special relativity. (E) None of the above is true.