Why is it “easier said than done” to ensure that the principle of informed consent is adhered to?

(A) It is not practicable to present every participant with all the information about the study
(B) Sometimes it is desirable to withhold certain pieces of information, such as the length of time an interview will take
(C) If the participants knew exactly what the researcher was intending to study, they might change their behaviour
(D) All of the above