Which of the Following Is a Drawback of Longitudinal Studies

Which of the following is a drawback of longitudinal studies of intelligence?

A) They are especially prone to the distortion of cohort effects.
B) People who remain in the study may be the healthiest and most well-educated.
C) The biases of the experimenter are more likely to distort the results than is true of other research methods.
D) All of these are drawbacks.