[单选题]
15.21: An object is launched from the ground a(pab rz2f5h2 vt an anil1j x9dy/ x)lgle of $60^{\circ}$ above the horizontal with a speed of 20.0 m/s. What is the magnitude of the average velocity of the object from just after launch until it reaches its highest vertical position during flight?
A. 13.7 m/s
B. 13.2 m/s
C. 10.0 m/s
D. 9.3 m/s
E. 8.7 m/s
