An externality is an unwarranted cost or benefit that affects... O the producer only
O the consumer only
O a third party
O both the producer and consumer​

a third party

Explanation:

A product can be defined as any physical object or material that typically satisfy and meets the demands, needs or wants of customers. Some examples of a product are mobile phones, television, microphone, microwave oven, bread, pencil, freezer, beverages, soft drinks etc.

In Economics, an externality could either be positive or negative depending on its effect on a third party.

A positive externality arises when the production or consumption of a finished product or service has a significant impact or benefits to a third party that isn't directly involved in the transaction.

On the other hand, a negative externality arises when the production or consumption of a finished product or service has a negative effect and/or impact (cost) on a third party.

This ultimately implies that, an externality is typically an unwarranted cost or benefit by a manufacturer or producer of goods and services that affects a third party.

u = 30

continuing your method

= +

subtract from both sides

= - thus

= -

= ⇒ u = 30

answer/// there is an inverse relationship between price and quantity demanded;