As a Performance Measure for Social Networks, the Click-Through Rate

As a performance measure for social networks, the click-through rate is

A) the percentage of recipients who have clicked on a link on the page to visit a specific site.
B) the percentage of e-mail recipients who buy the product after receiving a targeted message with an embedded link.
C) the percentage of customers who buy a product from the brand website.
D) the total number of people who have visited a brand's Facebook page and have become Facebook fans.
E) the total number of people who have purchased the product online divided by the number of people viewing the page.