Auto populate 2 variables in 3rd cell

I have a spread sheet that i'm creating for my company when placing pre-orders for new products and placing orders for current products.

I'm some what of a newbie to excel especially all the functions that are available in this program.

Basically, I have 2 columns, each with a drop down menu (validation) applied to about 20 rows. There are 6 options in column A's drop down menu, and 20 options in column B. I would need a defined variable in column C to auto-populate.

Any way of being able to do this? Thanks~!

or just link me to a thread where my question has already been answered or a solution is posted? Takes about 30 more seconds to do that instead of just ignoring the whole question at hand.