<image>As shown in the figure, this is a pie chart of whether eighth-grade students in a certain class know their parents' birthdays. Among them, A represents students who only know their father's birthday; B represents students who only know their mother's birthday; C represents students who know both parents' birthdays; D represents students who do not know either parent's birthday. Thus, in the class of 40 students, the number of students who know their mother's birthday is (__)
