## **Core Concept**
The attack rate is a measure used in epidemiology to calculate the proportion of individuals exposed to a potential cause (in this case, eating fruit salad) who develop a specific condition (food poisoning). It is calculated as the number of people who develop the condition divided by the total number of people exposed.

## **Why the Correct Answer is Right**
To calculate the attack rate, we first need to determine the total number of people who ate the fruit salad and then find out how many of them developed food poisoning. Given that 110 people attended and 40 didn't eat the fruit salad, 110 - 40 = 70 people ate the fruit salad. Since 55 people who ate the fruit salad didn't develop food poisoning, the number of people who developed food poisoning is 70 - 55 = 15. The attack rate is then calculated as the number of people who developed food poisoning (15) divided by the total number of people who ate the fruit salad (70).

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, without calculation, we cannot assume its validity. Let's assume it's a random value; the actual calculation will prove its incorrectness.
- **Option B:** Similarly, this option's correctness cannot be assumed without calculation. Given the numbers, let's assess if it matches our calculation.
- **Option C:** This seems to align with a calculation of 15/70, which simplifies to 3/14 or approximately 0.214 (rounded to three decimal places is 0.214). This matches our step-by-step calculation.
- **Option D:** This option does not match our calculated attack rate.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the attack rate is specifically used for outbreaks and helps in identifying the cause of the outbreak. It's crucial in food poisoning cases to quickly identify the source and take preventive measures.

## **Correct Answer:** C. 21.4%