What Is the formula of the permeance of the flux tube considering unit depth in the flux plotting technique?

a) permeance of the flux tube = permeability in the air * mean width of flux tube * mean length of flux tube
b) permeance of the flux tube = permeability in the air / mean width of flux tube * mean length of flux tube
c) permeance of the flux tube = permeability in the air * mean width of flux tube / mean length of flux tube
d) permeance of the flux tube =1/ permeability in the air * mean width of flux tube * mean length of flux tube