If x∈Ax \in Ax∈A and A⊆BA \subseteq BA⊆B, which of the following must be true?
x∉Bx \notin Bx∈/B
x∈Bx \in Bx∈B
B⊆AB \subseteq AB⊆A
{x}=B\{x\} = B{x}=B