Which of these expressions requires a variable? A
the expression for how much money Abe earns when he makes $15 an hour and works for three hours

B
the expression for the number of pencils Cody has if he starts with nine pencils and then gives four to Molly

C
the expression for the total number of cakes when Cindy brings one cake to class, and Bryan brings two cakes to class

D
the expression for how many birthday parties Pia has this June if she has one more birthday party than her brother