Nine-year-old Ricky has recently learned how to solve long division problems, and he still struggles with especially difficult problems. At his mother's request, Ricky helps his eight-year-old sister Lucy with the simple long division problems she must do for her math homework. Which one of the following is most likely to result? a. Ricky's own long division skills will improve because he is more motivated. b. Ricky will gain nothing from helping his sister because doing long division is outside his zone of proximal development. c. Ricky's own long division skills will decrease, because any mistakes that Lucy makes will "corrupt" his own mathematical thinking. d. By helping Lucy with her long division problems, Ricky will be able to practice using the central conceptual structure that underlies his mathematical thinking.