[单选题]
The kinetic energy of a proton is 80% of its total energysuj 2di 7(9ic r/6oraj. What is the momu)6 e1ckar-bcentum i1-k ccbruea) 6n $kg \cdot m/s$ 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. $2.5 \times 10^{-18} \; kg \cdot m/s$