[单选题]
An object's angular momengbir*lho tabr( u;6 :xp0 8nn/ti)yhx8z*x-tg5 +boarum changes by $20 \; kgm^{2}/s$ in 4.0 s. What magnitude average torque acted on this object?
A. $2.5 \; N \cdot m$
B. $5.0 \; N \cdot m$
C. $40 \; N \cdot m$
D. $80 \; N \cdot m$
