Age and experiences are confounded while using the cross-sectional design. This means that which of the following cannot be determined?
a. Whether differences across groups are due to age, experience, or the experimental manipulation
b. Whether findings for cohort groups are due to their similar experiences or similar age
c. Whether differences across groups are due to differences in assignment to the treatment conditions
d. Whether findings are due to the fact that age and life experience tend to be correlated