What Is the Key Limitation of Crossover Study Designs

What is the key limitation of crossover study designs?

A) Researchers using this design have to ensure there has been an appropriate waiting time before the second treatment can be implemented.
B) Each group has to be analyzed by group and not by the individual subjects themselves.
C) Sometimes the two treatments can interfere with each other or have negative effects for the participants.
D) If there is no difference between the study groups, you do not know whether that finding is true or if it could be based on other factors like measurement error.