In the Old Days, Milkmaids Were Exposed to Cowpox from Milking

In the old days, milkmaids were exposed to cowpox from milking the cows, and this fortuitously gave them some protection from smallpox. What was happening in the milkmaids?

A) They were receiving a dose of generic IgG from the sick cows.
B) They were effectively receiving a vaccination from a related virus.
C) They were heightening their overall immune response due to exposure to the cowpox and other cow parasites.
D) They had a stronger immune response from drinking the milk, which sent cow B cells circulating through their blood.