Agood that is excludable is one that someone can be prevented from using if she did not pay for it.
a. true
b. false

That statement is true.
You can see this type of good on the fishing ground.
In this type of business, an entity will rent up some sort of place for the people to freely fish in that place without paying. But, if those people catch a fish and want to take it, they have to paid an amount of money for that fish.

Clean air provides nitrogen