if-then function returning variable not defined

I'm trying to create a function where one variable selects whether run a circumference, area or volume function and the second variable is the radius for the selected function. However I keep getting error "c" not defined. I've tried defining the conditions inside and outside of the function 'typeSize' with no luck. '

import math
PI=math.pi

def typeSize(form, radius):
  if form == c:
        def cir(radius):            #circumference of a cirlce
            cCirc=2*PI*radius
            return cCirc
  elif form == a:
        def area(radius):           #area of a cirlce
            aCirc=PI*radius**2
            return aCirc
  elif form == v:
        def vol(radius):            #volume of a circle
          vCirc=4/3*PI*radius**3
          return vCirc

print(typeSize(c, 1))
`