A cyclist rides 12km due east in 30min and then rides 6km back toward the starting point in 20min. Which pair gives his average speed and the magnitude of his average velocity for the entire motion?
a) 7.2km/h,21.6km/h
b) 36km/h,12km/h
c) 21.6km/h,7.2km/h
d) 21km/h,3km/h