Multiple Subscriptions - Optional Levels

i’m developing a site.. we users can subscribe to content. I have a basic level and then 4 optional levels. so for example the basic is general english phrases , but then you can take any of the optional levels like business phrases, social phrases etc. So can I a) allow users to have multiple subscriptions so in theory they could have 5 subscriptions and b) how can I only show the optional subscriptions once the they have subscribed to the basic level