sorry i'm wrong.
B is correct.
From Purdue's OWL:
"1. Skim your paper, looking only for the seven coordinating conjunctions:
and, nor, but, so, for, or, and yet.
2. Stop at each of these words to see whether there is an independent clause (a complete sentence), on both sides of it. (For more help, see our handout on independent clauses.)
3. If so, place a comma before the coordinating conjunction. Examples:
She wanted to buy a new car, but she didn't have enough money to do so.
The wind blew fiercely, and the rain poured down.
Alaska was not the last state admitted into the US, nor does it have the lowest total population"
http://owl.english.purdue.edu/handouts/grammar/g_commaproof.html