Which of the following is an example of a positive externality?
a) Being given a free lunch from work, because such a thing exists
b) A concert being put on next door to you by a band that plays music you despise
c) When your car engine is running, you are able move from point A to point B even though there is some pollution
d) When your neighbors receive a flu shot, you benefit from their inability to transmit the illness to you.