An office messenger leaves the waiting area to deliver a parcel to a waiting client. The office messenger reaches the client's place after 12 min and returns to the waiting area. The messenger's distance from the waiting area as a function of time is shown below. What points represent constant and positive velocity?

• A. e and f
• B. a and d
• C. a and b
• D. b and c
• E. c and e
• F. a and f

Answer: (D)