We augment a red-black tree T by adding a field f to each node v storing some additional information. Which of the following augmentations could be maintained under insertions and deletions without affecting the O(log n)running time. (Hint: be careful incorrectly selected answers give negative points.) Select one or more: f stores the order-statistic of v. O b. f stores the number of nodes that are descendants of v. O c. f stores the maximum of all keys in the subtree rooted at v. O d. f stores the sum of all keys smaller than v