What's the difference between # and #! in an AJAX-driven web application?

None of the answers are correct

#! is AJAX-compatible, whereas # is not

#! tells the browser not to record user data such as browser history, whereas # will store user data

#! tells the browser that PHP is required, whereas # is not

#! is a common convention used to indicate that a site is AJAX crawlable

#! indicates that a site is CORS-enabled

# is AJAX-compatible, whereas #! is not

#! tells the browser that cookies are not required, whereas # required cookies to be enabled