What are the equations for the following steps in the mechanism for the reaction of fluorine with fluoromethane (ch3f) to form difluoromethane (ch2f2)?

a. F + CH3F → CH2F2 + HF
b. F + CH2F2 → CH3F + HF
c. CH3F + F → CH2F2 + HF
d. CH3F + F → CHF3