What is the formula for the outer cylindrical heat dissipating surface of the magnet coils?

• A. outer cylindrical heat dissipating surface = 3.14 * outer diameter of coil + axial length of coil
• B. outer cylindrical heat dissipating surface = 3.14 + outer diameter of coil + axial length of coil
• C. outer cylindrical heat dissipating surface = 3.14 / outer diameter of coil + axial length of coil
• D. outer cylindrical heat dissipating surface = 3.14 * outer diameter of coil * axial length of coil