Which of the Following Is NOT a Reason the Incentive

Which of the following is NOT a reason the incentive effects of OIP schemes are difficult to measure?

A) Cross-sectional data inherently involves comparisons of different workers at the same point in time.
B) It is impossible to discern whether the observed earnings differences in cross sectional data result from incentives to work harder or from systematic unobserved differences in workers who select into different earnings schemes.
C) Theoretically,workers with higher abilities will tend to select into OIP schemes.
D) Workers with higher ability are able to produce more output.
E) Workers do not increase effort under OIP schemes.