Which word and punctuation mark corrects the punctuation error in this sentence?

my appointment is at 3 p. m. with dr russell brown, m. d.

a.
p. m.,

b.
dr.

c.
m, d

Well, it isn't A because there should only be a period at the end of P.M. It isn't C because it has to be a period between M and D.

So, the answer has to be B.

Hope this helps!