...like YouTube that allow people to share content such as...
根据下文These sites allow people to stay in touch with friends,make new ones and join communities.Facebook is an example of this type.)这些网站允许人们与朋友保持联系,结交新朋友,加入社区Facebook就是这种类型的一个例子。)可知这里意思是讲述另外一种社交媒体是社交网站,选项C"Another ...