Which of the following statements with regards to search trees, B-trees, and B⁺-trees is correct?

a. An unbalanced B-tree allows for additional performance gain by reducing the height of the tree.

b. B-trees are a primary file organization method and directly impact the physical location of records.

c. Search trees are a good alternative to B-trees and B⁺-trees because they allow for better maintenance.

d. The performance gain of binary search trees over linear search trees becomes larger as the number of key values increases.