How do I solve this question and is the question after e. a hint?

A large music file sharing web site aims to analyze the musical preferences of

its users. For each user the web site administrators have a user name, email, age,

geographic location and a list of songs he or she downloaded. For each of the

following statistical questions suggest an appropriate way to divide the users into

clusters for a cluster sampling.

[a] The overall popularity of a certain artist.

[b] The likelihood of fans of Artist A to also like artist B.

[c] The amount of time a user stays on-line searching for new music.

[d] The number of songs a user downloads.

[e] The likelihood of a user buying a CD of the music he or she downloaded.

That is, should the users be clustered by age or email address to survey the number of songs they tend to download.