Concept:When two goods can replace each other, the demand for them is competitive demand.
Explanation:Postal services and e-mails are both used for sending messages.
When people increase the use of e-mails, they send fewer letters through postal services.
So, the demand for postal services falls because e-mails are being used instead.
This shows that the two services are substitutes.
Substitute goods compete with each other for consumers' demand.
Therefore, postal services and e-mails are said to be in competitive demand.
Answer:C. competitive demand.
