Clickthrough Counts Do Not Necessarily Result in an Accurate Count

Clickthrough counts do not necessarily result in an accurate count of individual users who use an online banner advertisement because:

A) privacy laws prevent advertisers from tracking users under age 16.
B) users can change the settings on their computer to prevent tracking.
C) software on the website cannot delineate between individual users.
D) there is no reliable way to tabulate clicks on web advertisements.
E) a single user could click on a particular banner ad several times.