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

Multiple Choice

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

A) According to Piaget, preoperational children cannot reason with variables.
B) Children in the concrete operational stage cannot reason with hypothetical entities such as variable.
C) Children are incapable of conservation and reversibility until the beginning of the formal operational period.
D) Dealing with variables requires postconventional thought, which does not usually appear until adolescence.