A charged particle enters a region of crossed electric and magnetic fields ($\vec{E}\perp\vec{B}$). For the particle to travel in a STRAIGHT line, the magnitudes must satisfy

1. In crossed fields, the electric force ($qE$) and magnetic force ($qvB$) act in OPPOSITE directions (by suitable choice of geometry). 2. For straight-line motion, the net force must be zero: $qE = qvB$. 3. Cancel $q$ and solve for $v$: $v = E/B$. 4. This is the principle of a VELOCITY SELECTOR — only particles with the specific speed $v = E/B$ pass through undeflected; others are filtered out. 5. Velocity selectors are used in mass spectrometers ahead of a magnetic mass analyser to ensure a monoenergetic beam. 6. Options B–D have wrong functional forms or dimensions. _Source: NCERT Class 12 Physics Part 1, Ch 4, §4.2.2 (Motion in Combined Fields — velocity selector), p. 6._