A technician needs to give users the ability to transfer files between mobile devices. The technician teaches the staff how to use a file transfer method in which the devices need to be in proximity with line-of-sight access to each other.
Which of the following technologies is the technician teaching to the staff?

a. IR
b. Bluetooth
c. Tethering
d. NFC

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)?