This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
A family consists of 2 parents & 6 children susceptible to measles. There occurs a primary case of measles and 3 secondary cases within a short period of time. Secondary attack rate is 
Select and explain. 
A. 60%, B. 38%, C. 67%, D. 50%
D: 50%
Answer: D: 50% 
Explanation: None