**Core Concept**
Incidence in epidemiology refers to the number of new cases of a disease or condition that occur within a specified time period among a population at risk. It is a measure of the risk of developing a disease in a given time period.

**Why the Correct Answer is Right**
To calculate the incidence of myopia in the town, we need to identify the population at risk, which is the number of people who were not myopic on January 1, 2011, and developed myopia by December 31, 2011. The number of new myopia cases that occurred in 2011 is given as 90. The population at risk can be calculated as the total population minus the number of people who were already myopic on January 1, 2011: 5000 - 500 = 4500. The incidence of myopia can be calculated as the number of new cases (90) divided by the population at risk (4500).

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely to be incorrect because it does not take into account the population at risk, which is essential for calculating incidence.

**Option B:** This option is likely to be incorrect because it uses the total population instead of the population at risk.

**Option C:** This option is likely to be incorrect because it does not provide a clear formula for calculating incidence.

**Clinical Pearl / High-Yield Fact**
When calculating incidence, it is essential to identify the population at risk, as this will give you an accurate measure of the risk of developing a disease in a given time period.

**Correct Answer:** B. 90/5000.