(阅读理解题)It is a great truth because once we truly understand and accept it, then life is no longer difficult. Most do not fully see this truth. Instead they complain about their problems and difficulties as if life should be easy. It seems to them that difficulties stand for a special kind of suffering especially forced on them or else on their families, their class, or even their nation. What makes life difficult is that the process of facing and solving problems is a painful one. Problems, depending on their nature, cause sadness, or loneliness or regret or anger or fear. These are uncomfortable feelings, often as painful as any kind of physical pain. And since life causes a lot of problems, life is always difficult and is full of pain as well as joy. Yet, it is in this whole process of solving problems that life has its meaning. Problems are the serious test that tells us success from failure. When we want to encourage the growth of human spirit, we encourage the human ability to solve problems, just as in school we set problems for our children to solve. It is through the pain of meeting and working out problems that we learn. As Benjamin Franklin said, "Those things that hurt, instruct ." It is for this reason that wise people learn not to fear but to welcome the pain of problems. (1). Which sentence best express the central point of the passage?