18. The current in a long solenoid varies according to I(t) = 4+ 6t² A. The solenoid has 800 turns/m and a radius of 2 cm. At t = 2 s find the magnitude of the induced electric filed at 　4 cm from the central axis. (A) 1.21x10⁻⁴ V/m, (B) 1.21 x10⁻⁴ J/(A*m), (C) 2.42x10⁻⁴ V/m, (D) 2.42x10⁻⁴ J/(C*m).