Logic gates are the building blocks of which system?

$A$ person applies a sine wave and a square wave to an $AND$ gate as shown in figures $(i)$ and $(ii)$. Assuming that both the voltages are applied in phase,the person observes the output at $E$ and $F$ on $(i)$ and $(ii)$,respectively. [Assume a minimum voltage of $5 \ V$ is equivalent to logic $1$].

The Boolean equation $\overline{A} \cdot \overline{B} = \dots$

The output $y$,when all three inputs are first high and then low,will respectively be

Find the correct combination of $A, B, C$ and $D$ inputs which can cause the $LED$ to glow.

In the given circuit as shown,two input waveforms $A$ and $B$ are applied simultaneously. The resultant waveform $Y$ is: