I have this multiplexer and decoder that I want to draw their truth table
So I figured out the answer:
O0 = A'B'
O1 = A'B
O2 = AB'
O3 = AB  

When the selection input of the multiplexer is 00, we have 0 => O0 so we do A'B' whenever we have 00 in the truth table for CD and so on...