A proton moving with a constant velocity passes through a region of space without any change in its velocity. If E and B represent the electric and magnetic fields respectively, this region of space may not have
a. E = 0 , B = 0
b. E = 0 , B ≠ 0
c. E ≠ 0 , B = 0
d. E ≠ 0 , B ≠ 0