The digits $0$ and $1$ are called bits or binary digits. A string b$1$b$2...$ bn with each bi

either $0$ or $1$ is an n$-$bit string or n$-$digit binary string. We use Bn to denote the set

of all n$-$bit strings. The number of $1$s in a given string is called the weight of the

string. We use Bn$,$k to denote the set of all n$-$bit strings of weight k$.$ Write down the

sets B$3$ and B$4,2$