"Cookies cannot reliably identify individual users."

Which of the following is the reason for the statement shown above?

a$.$

Internet service providers assign arbitrary IP addresses to devices accessing the Internet from the same network.

b$.$

Individual users use different browsers on the same computer to surf the Web.

c$.$

Cookies assigned by Web servers are prone to duplication.

d$.$

The U$.$S$.$ government has banned the use of cookies for user identification.

e$.$

The explosion in Internet$-$enabled mobile devices has resulted in an acute shortage of IP addresses.