Database Questions, please help

Hi Guys,

I have a series of questions that I am grappling with. If you can give me an idea, I will greatly appreciate it.

Thanks.

1. You are given a table (A, B, C) where A is a low-cardinality attribute, B is a high-cardinality attribute, C is a numerical variable. You're asked to produce cross-tabulation of sum of C by both A and B and show resultant table in Excel spreadsheet. Would you choose to show A as columnar dimension? Why?

2.What key mathematical property of a field is used in deciding if it should belong to
an index?