Why Do Externalities Arise

Why do externalities arise?

A) The costs of production are not borne by the producer
B) An economic activity imposes a burden on them who are not directly involved in it
C) An economic activity imposes a cost on them who are directly involved in it
D) The government produces such goods and services which are consumed by only a particular class of people
E) Goods of mass consumption are not produced as they do not yield profit for the producers