Consider a grid file on relation R(A,B) with both attributes in the range -100 to +100, such the the grid lines in both dimensions are located at positions that are multiples of 20. Suppose we want to find the points in R that are closest to the point P(40,60). If the query returns the point Q(50,50), how many other buckets we should look into to verify that Q is indeed the closest point to P?