Regarding the number of elements of sets $A$ and $B$
$$\begin{aligned} & s ( A - B ) = s ( B - A ) = s ( A \cap B ) \\ & s ( A \cup B ) = 24 \end{aligned}$$
the equalities are given.
Accordingly, how many elements does set A have?
A) 9
B) 12
C) 15
D) 16
E) 18