**Core Concept:** A randomized controlled trial (RCT) is a type of study design used in medical research to compare the effectiveness of two or more interventions. It involves random allocation of participants to different groups, and collecting data to determine the effects of the interventions on the outcomes of interest.

**Why the Correct Answer is Right:** In an RCT, all the statements are true, except for statement C, which states "all participants are selected from the same population." This is not accurate because RCTs can involve different populations in each group. The study participants are randomly assigned to the intervention and control groups, ensuring comparability between the groups.

**Why Each Wrong Option is Incorrect:**
A. False: "All participants are selected from the same population."
B. False: "All participants are selected from the same population."
D. True: All other statements are true, but this statement is incorrect due to the explanation above.

**Clinical Pearl:** When designing an RCT, it is essential to ensure that the study participants are randomly selected from different populations to avoid confounding factors that may affect the study's results. This helps to reduce bias and provide accurate conclusions regarding the effectiveness of the interventions being compared.

**Correct Answer:** . False: All participants are selected from the same population.