1.6 A B D A learner standing at a roadside records the frequency of sound waves produced by the siren of an ambulance. The ambulance is moving at constant velocity along a straight horizontal road. The frequency-time graph for the detected sound is shown below. Time (s) Which ONE of the following statements concerning the motion of the ambulance is CORRECT? C Detected frequency The ambulance ... (ZH) approaches the learner and then passes the learner. moves away from the learner, then turns and approaches the learner. approaches the learner, then turns and moves away from the learner. moves away from the learner and then stops. (2)​