听力原文： Two economists did the study: Andrew Oswald of the University of Warwick and David Blanchflower at Dartmouth College. They used information collected earlier on two million people from eighty nations. They found that people around the world seem to share an emotional design in life. That design is shaped like the letter U. Levels of happiness are highest when people are young and when they are old. In the middle, most people's happiness and life satisfaction levels drop. Professor Oswald says some people suffer from midlife depression more than others. But it happens to men and women, to single and married people, to rich and poor and to those with and without children. According to the expert's study, the level of happiness in which period of life is the lowest?