Construct a circuit that computes the product of the two-bit integers $\left(x_{1} x_{0}\right)_{2}$ and $\left(y_{1} y_{0}\right)_{2} .$ The circuit should have four output bits for the bits in the product.
Use NAND gates to construct circuits with these outputs.
$$
\begin{array}{ll}{\text { a) } \overline{x}} & {\text { b) } x+y} \\ {\text { c) } x y} & {\text { d) } x \oplus y}\end{array}
$$