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: Dental
Which one of the following is an Index of communicability of an Infection? 
Select and explain. 
A. Carrier rate, B. Prevalence rate, C. Secondary attack rate, D. Primary attack rate
C: Secondary attack rate
Answer: C: Secondary attack rate 
Explanation: An  important  measure  of communicability is Secondary attack rate.
Secondary attack rate (SAR)  is defined as "the number of exposed persons developing the disease within the range of the  incubation  period,  following  exposure to the primary case" .  It is given by the formula :