What Is the Primary Problem with Interpreting the Results of Cross-Sectional

What is the primary problem with interpreting the results of cross-sectional studies of intelligence?

A) People may remember the questions from one test session at the time of the next test session.
B) Cohort effects, rather than age, may be the cause of differences in performance.
C) The sample is typically not representative of the population.
D) The sample may change across time.