[单选题]
An object's angular momentum ci 6 xgj 6;2 qcc+wmrr6*+jsnwhhany 3k7u:+rw be 9mzus4 b(c;ipfges 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. $80 \; N \cdot m$
