I have this problem that I cannot think of a way to solve:

I have a few categories of items that I wish to select in Excel, using two
separate lists. For example, I have the following categories:

1. Systems
2. People
3. Resources

And in each category, I have a few items:

1. System - IT, Process, Inventory
2. People - Cost, Training
3. Resources - IT, Fixed Assets

I want to create two lists, such that the first list will be on the three
main categories, but when I select a particular cateogry, the second list
will only reflect the items under that particular category. How do I do
that? Thanks.