Freda, Sam, and Liz are eighth-grade students with learning disabilities. Which one of the following statements best illustrates the concept of intraindividual variation?
a. Freda scores high in math, but not in spelling.
b. In comparison to eighth graders nationally, Sam scores lower in reading and written expression.
c. In comparison to Freda and Sam, Liz has higher achievement in math.
d. Liz has lower reading scores than any of the nondisabled students in her class.