At certain times in life, we all face changes that many social scientists consider to be natural stages. Take a newborn, William, as an example. The first few months of his life ＿＿（16）＿＿ in a limited environment. In fact, he won't develop an awareness of his surroundings or truly enter the world ＿＿（17）＿＿ he's begun to walk and talk. William will be spending the next few years discovering the world around him. Later, as he approaches adolescence, his ideas and feelings, ＿＿（18）＿＿ his body, will begin to change. His outlook on the world will broaden. When he has passed through his teenage years, he'll face the choices of young adulthood: marriage, family, career. Through the thirties, his life may go smoothly, but ＿＿（19）＿＿ another "passage" will approach near "midlife," as he reaches his forties. At that point, time will " ＿＿（20）＿＿ ." Suddenly, he will find himself in his sixties or seventies, fast approaching the end of his life.