Answer: Option B
A URL (Uniform Resource Locator) is a string of characters that provides a way to identify and locate a resource on the Internet. It consists of several components, including a protocol identifier, domain name, and path to the resource. However, not all URLs contain all of these components.
Let's examine each option to determine which must be contained in a URL:
A. A protocol identifier: This is the scheme used to access the resource, such as "http://" or "https://". While most URLs do include a protocol identifier, it is not always necessary. For example, many web browsers will automatically assume "http://" if no protocol is specified.
B. The letters, www.: This is not strictly necessary, but it is commonly used to indicate a website. "www" stands for "World Wide Web" and was originally used to distinguish web pages from other types of content on the Internet. However, many websites now omit "www" from their URLs, so it is not a requirement.
C. The unique registered domain name: This is the main part of the URL and is used to identify the server that hosts the resource. It is essential for locating the resource and is always included in a URL.
D. www. and the unique registered domain name: This option is not necessary since "www" is not a required part of a URL, as discussed above.
E. None of the above: As explained above, option B is correct, so this option is incorrect.
In summary, a URL must contain a protocol identifier (usually) and a unique registered domain name (always), but the letters "www" are not necessary.