In One Elementary School Curriculum,mathematical Variables First Appear in the Sixth

In one elementary school curriculum,mathematical variables first appear in the sixth grade.Why are they not introduced in the fifth grade?

A) Children must be in the preoperatonal stage to handle hypothetical variables.
B) Children must be in the formal operational stage to handle hypothetical variables.
C) Children must be in the concrete operational stage to handle hypothetical variables.
D) Children must be in the postconventional stage to handle hypothetical variables.