Supplementary Exercises*: Nets: Problem 1 Solution
Working problems is a crucial part of learning mathematics. No one can learn topology merely by poring over the definitions, theorems, and examples that are worked out in the text. One must work part of it out for oneself. To provide that opportunity is the purpose of the exercises.
James R. Munkres
Given two elements the one that is greater than both is: (a) the greater one (any pair of elements are comparable), (b), (d) the union of a pair of subsets, (c) the intersection of a pair of subsets.