This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
Among the above characteristics, which are used to measure the ability of biological agents to induce clinically apparent illness? 
Select and explain. 
A. 1 and 2, B. 2 only, C. 3 only, D. 2 and 3
A: 1 and 2
Answer: A: 1 and 2 
Explanation: .