Fill cell based on values entered from two drop downs

I'll apologize now, I'm a relative Excel novice. Anyhow, I have a risk assessment worksheet in Sheet 1 set up by someone else. I'm stuck with it for the most part and I can't make major changes to layout. Here's what I have:

Column B has a drop down list for risk severity with 5 levels of severity (catastrophic, critical, major, minor, negligible). Column C has a drop down list for risk probability with 5 levels of probability (frequent, likely, occasional, seldom, unlikely). There's a table in Sheet 2 (I can move it to the bottom of Sheet 1 if necessary) that has a code based on severity and probability which goes into Column D. 25 values total. Right now it's a drop down list that I have to manually select. I'd like to save time by having Column D auto fill when I select the severity and probability. I suspect this is super easy, but I'm no Excel master.

Thanks for the help!