In Coase's Confectioner and Doctor Example,what Was the Externality

In Coase's confectioner and doctor example,what was the externality?

A) The doctor building his office so close to the confectioner.
B) The vibrations generated by the confectioner's production process.
C) The large number of people entering the confectioner's factory thinking it was the doctor's office.
D) The screams of the doctor's patients disrupted factory workers from performing their duties.