Each scenario below involves an externality. For each, identify the externality and explain whether it is “likely” or “not likely” that private bargaining will lead to a socially optimal outcome.

a. Conducting scientific research that is likely to spark other innovations. 

b. Putting together an excellent music playlist. 

c. Mowing your lawn in the middle of the night. 

d. Producing steel that results in pollution affecting millions of residents in the surrounding area.