7. List all two-element subsets of a five-element set.
7. Let's take the five-element set $\{A ; B ; C ; D ; E\}$. We will list its two-element subsets:

$\{A ; B\},\{A ; C\},\{A ; D\},\{A ; E\},\{B ; C\},\{B ; D\},\{B ; E\},\{C ; D\}$, $\{C ; E\},\{D ; E\}$. In total, we have 10 subsets.