State whether the following are either Positively External, Negatively External, or Not External.

State whether the following are either Positively External, Negatively External, or Not External.

1) doctors earn income by charging for flu shots

2) flu shots are less expensive to an individual than the cost of catching the flu

3) flu shots reduce the likelihood of others catching the flu

4) flu shots reduce that individual's sick days allowing them to earn more income