[单选题]
The kinetic energy of a protonls h/3n24w* g7lk xor.mgkj 0f is 80% of its total energy. What is the momen63lse c3/lro rtum3c /errs l6o3l in $kg \cdot m/s$ 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. $2.5 \times 10^{-18} \; kg \cdot m/s$