A problem with cross-sectional research is that: A. cohort differences can make it difficult to differentiate between developmental change and contextual differences. B. the results tend not to be worth the large investment. C. the most intelligent people are most likely to drop out of the study. D. retesting leads older subjects to do better.

A. cohort differences can make it difficult to differentiate between developmental change and contextual differences.          

Explanation:

Cross-sectional research: In psychological research, the term "cross-sectional research" is described as one of the different research methods or study whereby different subjects or participants of distinct ages are being compared simultaneously. Cross-sectional research methods are being utilized in various areas including social sciences, developmental psychology, education, etc.

In the question above, the given statement represents the correct answer as A.

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