Which of the Following Statements Reflects a Situation in Which

Which of the following statements reflects a situation in which there are external benefits?

A) John paints his house and cleans his paintbrushes in the stream.
B) John pays 5 percent of his income as taxes.
C) John's decision to get vaccinated for smallpox reduces the chances that his neighbor Pete will get smallpox.
D) John sells his car to his neighbor Pete at half the first-hand price.