What is the formula of the inner cylindrical heat dissipating surface?

• A. inner cylindrical heat dissipating surface = length of mean turn * axial length of coil
• B. inner cylindrical heat dissipating surface = 2 *length of mean turn * axial length of coil
• C. inner cylindrical heat dissipating surface = length of mean turn / axial length of coil
• D. inner cylindrical heat dissipating surface =1 / length of mean turn * axial length of coil