⟩ How many 21 multiplexers do we need to make a 2 input XOR gate?
A xor B= A'B + AB'
hence, if A=0, then the output is B
if A=1, then the output is B'
so we need only ONE mux (2:1), which is controlled by A, and its inputs are B --> mux_in_0
B' --> mux_in_1