[单选题]
The kinetic energy of a proto9: 4nvj fox.sn*0 c21un2ccn cd r,gksn is 80% of its total enel+- i6fw8ifr drgy. What is the momentum inl -f 8+wr6fdii $kg \cdot m/s$ of the proton?
A. $5.0 \times 10^{-19} \; kg \cdot m/s$
B. $2.5 \times 10^{-19} \; kg \cdot m/s$
C. $5.0 \times 10^{-18} \; kg \cdot m/s$
D. $2.5 \times 10^{-18} \; kg \cdot m/s$