If $A$ and $B$ are two sets such that $A \subset B$,then what is $A \cup B$?

State whether the following statement is true or false. Justify your answer.
${2, 6, 10, 14}$ and ${3, 7, 11, 15}$ are disjoint sets.

If $A = \{3, 6, 9, 12, 15, 18, 21\}, B = \{4, 8, 12, 16, 20\}, C = \{2, 4, 6, 8, 10, 12, 14, 16\}, D = \{5, 10, 15, 20\};$ find $D - C$.

$A - (A - B)$ is

If $U = \{1, 2, 3, 4, 5, 6, 7, 8, 9\}$,$A = \{2, 4, 6, 8\}$ and $B = \{2, 3, 5, 7\}$,verify that $(A \cup B)^{\prime} = A^{\prime} \cap B^{\prime}$.

If $P, Q$ and $R$ are subsets of a set $A$,then $R \times (P^c \cup Q^c)^c =$