## **Core Concept**
The question pertains to the properties of a normal distribution, specifically in the context of blood pressure (BP) measurements. In a normal distribution, about 68%, 95%, and 99.7% of the values lie within one, two, and three standard deviations of the mean, respectively.

## **Why the Correct Answer is Right**
Given that the mean systolic BP is 120 mm Hg and the standard deviation is 10 mm Hg, we can apply the 68-95-99.7 rule:
- About 68% of people will have BP between 110-130 mm Hg (mean ± 1 standard deviation: 120 ± 10).
- About 95% of people will have BP between 100-140 mm Hg (mean ± 2 standard deviations: 120 ± 2*10).
- About 99.7% of people will have BP between 90-150 mm Hg (mean ± 3 standard deviations: 120 ± 3*10).

## **Why Each Wrong Option is Incorrect**
- **Option A:** 68% of people have BP between 110-130 mm Hg. This statement is correct based on the calculation but let's evaluate all options.
- **Option B:** 95% of people have BP between 110-130 mm Hg. This is incorrect because 95% of people will have BP between 100-140 mm Hg, not 110-130 mm Hg.
- **Option D:** 99% of people have BP between 100-140 mm Hg. This is incorrect because 99.7% of people will have BP between 90-150 mm Hg, and the range 100-140 mm Hg does not correspond to 99%.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is the 68-95-99.7 rule for normal distributions, which helps in quickly estimating the range of values within which a certain percentage of the population falls. This rule is crucial in understanding and interpreting various clinical and laboratory measurements.

## **Correct Answer: C.**