Question: What happens when a PSS client sends an HTTP POST message with an XML bookmark request to the bookmarking server with only the `ContentURI` attribute set?
Option 1: The bookmarking server returns all private bookmarks belonging to the UserID associated with the PSS client.
Option 2: The bookmarking server returns all public bookmarks for the content identified by the `ContentURI`.
Option 3: The bookmarking server sends back an HTTP error due to insufficient attributes in the XML bookmark request.
Option 4: The bookmarking server returns a single bookmark identified by a default BookmarkID associated with the `ContentURI`.
Explanation: According to the bookmark retrieval protocol, if only the `ContentURI` attribute is set in the XML bookmark request, the bookmarking server retrieves all public bookmarks for the corresponding content. This behavior is specifically designed for accessing publicly available bookmarks without requiring user identification or specific bookmark IDs.