Quiz: Closest Pair

In the closest pair algorithm, why is the strip width $2\delta$ where $\delta$ is the minimum distance found so far?

1. A.To ensure at least one point is in the strip
2. B.It's required by the Master Theorem
3. C.It balances the left and right subproblems
4. D.Points farther than $\delta$ from the dividing line can't form a closer pair