A proton (mass 1.67x10^-27 kg, charge + 1.6x10^-19 C) travels at 10^4 m/s to the left through a uniform magnetic field of magnitude 0.2 T, which points toward the top of the page. What is the direction of the force on the proton?
a) up
b) down
c) out of the page
d) into the page