Here is the completed paragraph with the correct words:
How old are you? (1) Are you in your twenties or thirties? If your answer is 'yes', then you probably (2) feel stressed and maybe a little depressed about many things in (3) your life. However, researchers studying happiness found out something quite interesting. (4) The researchers recently interviewed 1,546 people between the ages of twenty and one hundred. (5) They learned that older people are much happier (6) than younger people. This information surprised the researchers (7) because older people have many health problems. The researchers are now (8) asking themselves, "Why are older people happier with (9) their lives?" There is no one, clear answer to this question. The answer may be, however, that older people have learned how to (10) think differently. For example, a younger person might (11) see a problem as a very big and very serious one. A person who is fifty or sixty, however, might view that same problem as a small problem. As Professor Jeste, at the University of California, says, "a lot (12) of big things become little" for older people. So, if you are young and a little unhappy now, don't worry. You will probably feel happier when you are older.