WeChat, a firm that owns an online mobile phone gaming application firm, offers number of games at zero cost to all users with basic features, under a plan called $$Basic Game Console$.$ For $$4\mathrm{.}25$ per month, WeChat offers upgrade with additional features such as priority online support, opportunity for network gaming, and an ability to save games rather than start them over. Which of the following revenue models best describes Dropbox$$s business model?