Consider the sets $\phi, A=\{1,3\}, B=\{1,5,9\}, C=\{1,3,5,7,9\}$. Insert the symbol $\subset$ or $\not\subset$ between the following pair of sets: $A \dots C$.

(A) set $A$ is a subset of set $C$ (denoted as $A \subset C$) if every element of $A$ is also an element of $C$.
Since the elements of $A$ are ${1, 3}$ and these elements are present in $C = \{1, 3, 5, 7, 9\}$,it follows that $A \subset C$.