Which of the Following Is Not a Difficulty of Running

Which of the following is not a difficulty of running an RCT?

A) It is difficult to know whether improvements in treated clients were due to the new intervention or simply the passage of time
B) Drop-out rates are often very high
C) It is difficult to make sure that outcome assessments are objective and unbiased
D) The researcher's excitement about an intervention may result in that intervention doing better than it's comparison