51. A long solenoid has 5000 turns per meter and a cross-sectional area of 2.0×10⁻⁴ m². A larger secondary coil with 500 turns, which has a cross-sectional area of 5.0×10⁻⁴ m², and a total resistance of 2π Ω is placed outside and coaxial with the central part of the solenoid. The current in the solenoid drops uniformly from 4.0 A to 0 A in a time interval of Δt = 0.1 s. What is the magnitude of the induced current in the external coil during this time interval? (Given: permeability constant μ₀ = 4π × 10⁻⁷ T·m/A) (A) 4.0 mA (B) 5.0 mA (C) 6.0 mA (D) 7.0 mA (E) 8.0 mA