The following circuit is a $2-$to$-4$ decoder.

Assuming A and $B$ are two buttons. $"A=0"$ means push button A down and $"A=1"$ means pull

button $A$ up$.$ Assuming each output is connected to a light bulb. $"$Q$0=0"$ means light is off

and $"$Q$0=1"$ means light is on$.$

To turn on light Q$0$ only$\_($Q$1,$ Q$2,$ and Q$3$ are all off$),$ which button$($s$)$ should be pushed

down?