Compare Columns and return a value from third column

Hello,

I need to compare 2 columns and fill data from a third column. Here is the scenario.

A B C D
1 1 X =X
2 3 Y =Z
3 2 Z =Y
4 4 A =A
5 9 B =D
6 8 C =?
7 5 D =?

I want the value of column A to match a value in column B, and if there is a match, I want to know what is in Column C in the matching row from B.

I hope that is enough info.

THanks,