A、To keep the ghosts happy. B、To avoid being recognized by ghosts. C、To let the spirits roam among the living. D、To form their o

In a recent study of six- and seven-year-olds in the Philadelphia area, Penn State psychologist Cindy Dell Clark found that most parents underestimate just how terrifying Halloween can be for young kids.
    Two thousand years ago, Celts living in what is now the United Kingdom celebrated their new year at the end of October. During these days of transition from the end of summer harvest to the beginning of winter, spirits were thought to roam among the living.
    The modern customs of candy and costume are rooted in medieval England. To avoid being recognized by the visiting spirits, people would dress up in masks whenever they left home. Bowls of food were placed outside to keep the ghosts happy. The practices have gradually changed into Halloween as it is known today, with parents encouraging their own little ghosts and goblins to haunt the neighborhood.
    There have been few studies to examine how the holiday affects children. Child psychologists generally caution parents that the fright of some aspects of Halloween can be too much for the very young, and advise adults to keep a close eye on children and remind them of what is real and what is not. For most kids, at an age when they’re often not included in family funerals or witness to grave illnesses, Oct. 31 is often their first introduction to the subjects.
    Halloween also provides an opportunity for adults to confront usually uncomfortable topics like death. Kids as young as six and seven, however, don’t differentiate between real death and the store-bought skeleton figures hanging in the trees.
19. What did psychologist Cindy Dell Clark find?
20. Why did people dress up in masks in medieval England?
21. What does the speaker say about the uncomfortable topics like death?