Bob loves to go surfing. Bob is also a student. On Saturday, Bob looks out the window and sees that it is a beautiful day, a perfect surfing day. Bob remembers that he has a Chemistry test on Monday and that he needs to study for his test. Bob decides that even though he would love to go surfing, he will use his Saturday to student for the Chemistry exam so that he can get an A in the class. The term used to describe that Bob has given up a day of surfing for something he desires (an A on the Chemistry test) is