47. A beam of protons moves parallel to the x -axis in the positive x -direction, as shown, through a region of crossed electric and magnetic fields balanced for zero deflection of the beam. If the magnetic field is pointed in the positive y -direction, in what direction must the electric field be pointed?
(A) Negative y-direction
(B) Positive z-direction
(C) Negative z-direction
(D) Negative x -direction
