Concept:The image formed by a plane mirror moves with the same speed as the girl, so the separation between them closes at the sum of their speeds.Explanation:The girl walks towards the mirror at v=4.0m/s.Her image moves towards the mirror at the same speed, 4.0m/s.Thus, the girl and her image approach each other.The rate at which the distance between them reduces is given by:vreduction=4.0+4.0=8.0m/s.So the separation between the girl and her image decreases at 8.0m/s.Answer:C. 8m/s