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 fourth or fifth grade

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