Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

A Grothendieck topology defines a sieve as a collection of subsets of a set with this property that themselves have this property. The Lindehf condition for a space requires every cover of this type to have a countable subcover. Generally, two points are "separated" if each point is contained in a set of this type that doesn't contain the other. In the particular point topology, all sets that contain an arbitrary point are of this type. A subset U of a metric space has this property if every point therein has a neighborhood inside U. Sets have this property in R^n when every point is an interior point. For 10 points, name this topological property, possessed by a set if its complement is closed.
Science