$($DNF$,15$ points$)$ A literal is a Boolean variable or its complement. A minterm of the Boolean

variables x$1,$ x$2,...,$ xn is a Boolean product y$1$y$2$ yn$,$ where yi $=$ xi or yi $=$xi

$.$ Hence, a minterm

is a product of n literals, with one literal for each variable. For example, x$1$x $2$x$3$x $4$ is a minterm when

n $=4.$