What Is the Intended Result of a Distributed Denial of Service

What is the intended result of a Distributed Denial of Service (DDOS) attack on a web site?

A) To change the content of the site to prevent it from providing further services
B) To distribute the content of the site to users who are denied access to its service
C) To infect the site to distribute a virus to users, which denies them access to further services
D) To lock users out of the site by exhausting the sites resources