[单选题]
An object's angular moment/0a1.f r(ouwbd*y qh w.9shvn wwv 95dum+s5fr6p 1pr nwo3nsyszd8 8c( 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. $80 \; N \cdot m$
