Which of the following is a correct statement? a. the include nulls keywords can be included in the argument for the count function to include null values in the results. b. the null keyword can be included in the argument of the count function to include null values in the results. c. an asterisk can be used as the argument for the count function to include null values in the results. d. the all keyword can be included in the argument of the count function to include null values in the results.

Suppose there is a relation r(a, b, c) with a b+-tree index with search keys (a, b).1. what is the worst-case cost of finding records satisfying 10 < a < 50 using this index, in terms of the number of records n1, retrieved and the height h of the tree? 2. what is the worst-case cost of finding records satisfying 10 < a < 50 and 5 < b < 10 using this index, in terms of the number of records n2 that satisfy this selection, as well as n1 and h defined above? 3. under what conditions on n1 and n2, would the index be an efficient way of finding records satisfying the condition from part (2)?