In a certain mass spectrograph, an ion beam passes through a velocity filter consisting of mutually perpendicular fields E⃗ and B⃗ . The beam then enters a region of another magnetic field B⃗ ′ perpendicular to the beam. The radius of curvature of the resulting ion beam is proportional to: A) EB'/B B) EB/B' C) BB'/E D) B/EB' E) E/BB'