[单选题]
A proton moving at $4.0 \times 10^4 \; m/s$ horizontally enters a region where a magnetic field of 0.13 T is present, directed vertically downward. What force acts on the proton?
A. zero
B. $1.4 \times 10^{-16} \; N$
C. $5.2 \times 10^{-16} \; N$
D. $8.3 \times 10^{-16} \; N$
