Which of the Following Studies Would Be Impossible to Answer

Which of the following studies would be impossible to answer by means of a cross-sectional study?

A) At what age do most children first understand the concept of conservation of number?
B) Do sixth-grade children generally like music different from what first-grade children like?
C) Do the children who are the best artists at age 6 remain outstanding at art as they grow older?
D) Do older children generally have a better ability to memorize facts than do younger children?