In the context of web development, what does it mean if a header is present on the requested resource?
A. The resource has metadata or additional information associated with it, which can be used for proper handling or processing of the resource.
B. The resource is encrypted and cannot be accessed without proper authorization.
C. The resource is missing and cannot be located on the server.
D. The resource is being updated, and no action can be performed until the update is complete.