According to Piaget, which of the following would a concrete operational child be unable to do?

A) Add 2 plus 3 and get 5.
B) Solve if A plus B equals 5, what is A?
C) Think about something from someone else's perspective.
D) Think about objects they have seen before.

Piaget's theory of cognitive development suggests that children in the concrete operational stage (ages 7 to 11) can perform operations on concrete objects but struggle with abstract thinking, which is required to solve for a variable in an equation like "if A plus B equals 5, what is A?" This type of problem requires abstract and hypothetical reasoning, which is more characteristic of the formal operational stage.