Which of the following is an example of a positive externality?

a. Bob mows Hillary’s lawn and is paid $100 for performing the service.

b. While mowing the lawn, Bob’s lawnmower spews out smoke that Hillary’s neighbor Kristen has to breathe.

c. Hillary’s newly cut lawn makes her neighborhood more attractive.

d. Hillary’s neighbors pay her if she promises to get her lawn cut on a regular basis.