An Airplane Is Flying North at 500 Km/h

Multiple Choice

An airplane is flying north at 500 km/h. It makes a gradual 180 turn at constant speed, changing its direction of travel from north through east to south. The process takes 40 s. The average acceleration of the plane for this turn in km/h.s is:

A) 12.5 km/h.s, north
B) 12.5 km/h.s, east
C) 12.5 km/h.s, south
D) 25 km/h.s, north
E) 25 km/h.s, south