1.  Choose that set of numbers from the four alternative sets, that is similar to the given set.
Given set : (9, 15, 21)

A. (10, 14, 16)
B. (7, 21, 28)
C. (5, 10, 25)
D. (4, 8, 12)

Answer: Option D

Explanation:

In each set, 1st number + 3rd number/2 =2nd number.