You Want Exactly Nine Gallons of Water, but You Have

You want exactly nine gallons of water, but you have only a seven-gallon bucket and a five-gallon bucket. Although you, at first, do not see how these two buckets can help, you realize that you can use them in the following way. You fill the seven-gallon bucket, then pour it in the five-gallon bucket, leaving two gallons in the seven-gallon bucket. You then pour out the five-gallon, pour the two gallons of water in the five-gallon, and refill the seven-gallon bucket, and you have a total of nine gallons of water. To solve this problem, you had to use

A) selective comparison.
B) selective attention.
C) selective encoding.
D) selective combination.