Consider sets $A$, $B$, $C$, and $D$ such that $B$ is a subset of $A$, $C$ is a subset of $B$, and $D$ is a subset of $C$. Whenever $x$ is an element of $B$, $x$ must be an element of:

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Based on the information provided, our sets may look like this:

If $x$ is in $B$, it must be in $A$ as well since every element in $B$ is also in $A$.