Happiness can be described as a positive mood and a pleasant state of mind. According to recent investigations, sixty to seventy percent of Americans consider themselves to be moderately happy, one in twenty persons feel very happy. Psychologists have been studying the factors contributing to happiness. It is not predictable nor is a person in an apparently ideal situation necessarily happy. The ideal situation may have little to do with his actual feelings.
A good education and income are usually considered necessary for happiness. Though both may contribute, they are only chief factors if the person is seriously under - educated or actually suffering from lack of physical needs. The rich are not likely to be happier than the middle-in-come group or even those with very low incomes. People with college education are somewhat happier than those who didn’t graduated from high schools, and it is believed that this is mainly because they have more opportunity to control their lives. Yet people with a very high income and a college education may be less happy than those with the same income and no college education.
It should be noted that people quickly get used to what they have, they are happiest when they feel they are increasing their level, no matter where it stands at a given time. The best formula for happiness is to be able to develop the ability to tolerate frustration, to have a personal involvement and commitment, and to develop selfconfidence and to selfesteem.
The italicized word " they " in the second paragraph refers to（）。
A. recent investigations
B. psychologists
C. education and income
D. mood and state of mind
