Set Theoretical Relations – Lecture 3 (Topology)

In the lecture's indexed-collection notation, if D is a set and for each d in D there is an associated set S_d, what role does D play?

1. A.D is the union of all the S_d sets
2. B.D is the indexing set, and C is the collection of sets indexed by D
3. C.D must itself be a set of sets
4. D.D is the intersection of all the S_d sets