This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
[{'content': 'When a drug is evaluated for its usefulness in controlled conditions, it is termed as a trial signifying- \nSelect and explain. \nA. Efficacy, B. Effectiveness, C. Efficiency, D. Effect modification', 'role': 'user'}, {'content': 'Answer: A \nExplanation: .', 'role': 'assistant'}]