A 050-M Long Solenoid Consists of 1 000 Turns of Copper

A 0.50-m long solenoid consists of 1 000 turns of copper wire wound with a 4.0 cm radius. When the current in the solenoid is 18 A, the magnetic field at a point 1.0 cm from the central axis of the solenoid is

A) 0.090 mT.
B) 0.36 mT.
C) 23 mT.
D) 36 mT.
E) 45 mT.