An unlabeled hierarchical diagram of various astronomical bodies is shown below. the labels a, b, c, and d can be used to represent the milky way, earth, universe, sun, and solar system.

an image of four circles nestled one within the other is shown. the lower boundary of all four circles touch one another. the largest and the outermost circle is labeled a. the next circle in order is labeled b. the next circle is labeled c. the innermost circle is labeled d.

part 1: which four astronomical bodies would you choose to represent the four labels in the diagram?
part 2: which astronomical body, other than the ones given, could also represent the label d?