Christina used the following expression to calculate the average ages of her three program users: average = age1 + age2 + age3 / 3 however, her output is not correct. what did she do wrong? a. she did not use parentheses around the age values to indicate the proper order of operations. b. she did not use quotation marks around the age values to indicate the proper order of operations. c. she should have used the multiplication symbol instead of the division symbol. d. she should have used a string function to perform the calculation.