Two sets $A$ and $B$ are disjoint if and only if

Write down all the subsets of the following set: $\{a\}$

Let $A = \{1, 2, 3, 4, 5, 6\}$. Insert the appropriate symbol $\in$ or $\notin$ in the blank space:
$4 \dots A$

Let $S = \{1, 2, 3, \ldots, n\}$ and $A = \{(a, b) \mid 1 \leq a, b \leq n\} = S \times S$. $A$ subset $B$ of $A$ is said to be a good subset if $(x, x) \in B$ for every $x \in S$. Then,the number of good subsets of $A$ is

Which of the following is an example of the null set?
$\{ y : y \text{ is a point common to any two parallel lines} \}$

If $A = \{1, 2, 3, 4, 5, 6\}$, then which of the following statements is false?