"Cookies Cannot Reliably Identify Individual Users

"Cookies cannot reliably identify individual users." Which of the following is the reason for the statement shown above?

A) Cookies assigned by Web servers are prone to duplication.
B) The US government has banned the use of cookies for user identification.
C) The explosion in Internet-enabled mobile devices has resulted in an acute shortage of IP addresses.
D) Internet service providers assign arbitrary IP addresses to devices accessing the Internet from the same network.
E) Individual users use different browsers on the same computer to surf the Web.