Which one of the following is not a valid enumerated type declaration?

a. enum Iron { Melting = 2795, Boiling = 4982 }
b. enum Speed Limit { City = 35, Highway = 60 }
c. enum Temperature { Cold = 20, Hot = 90 }
d. enum Voltage { Orange = 3.3, Green = 5.0 }

d. enum Voltage { Orange = 3.3, Green = 5.0 }