Cross-sectional area ratio controls drift speed scaling in parallel circuit Steps: - Diameter of R1 is half that of R2, so A1 / A2 = (1/2)^2 = 1/4, thus A2 = 4 A1. - R2 = 3 R1 implies L2 / L1 = 3 × (A2 / A1) = 3 × 4 = 12 using R ∝ L / A. - In parallel, same V, so I2 = I1 / 3. - Drift speed v_d ∝ I / A, so the area factor of 4 scales the speed ratio when combined with current. Why B is correct: - The area ratio A2 / A1 = 4 from the diameter squared, directly tying to Ohm's law and drift speed formula v_d = I / (n e A) where larger A reduces v_d by factor 4 for comparable currents. Why the others are wrong: - A: 3/4 ignores the squared diameter relation for area. - C: 6 assumes linear diameter scaling instead of quadratic for area. - D: 1/12 confuses the full ratio v_d2 / v_d1 with the area factor alone. Final …