Course Test: Topology (2020)

Under the axiomatic set theory developed in this course, the proposition 'x ∈ y' is a well-formed proposition exactly when:

1. A.x is a finite set and y is any kind of set.
2. B.Both x and y are themselves sets.
3. C.x is an element of some universe and y is a set.
4. D.y is a class containing x as a member of it.