Question 1
In a metric space, what is the most common defining idea of an open set?
Question 2
Which of the following is an open interval in $\mathbb{R}$ with the usual metric?
Question 3
In a metric space, why is the empty set considered open?
Question 4
Why is the whole space always open in a topological space?
Question 5
Which statement best describes the relationship between open sets and closed sets?
