## Core Concept
The core concept being tested here is the calculation of **incidence rate** of a disease, specifically myopia in this case. Incidence rate refers to the number of new cases that develop in a specified time period among a population at risk. It's usually expressed as a rate per 100 or 1000 population per unit of time.

## Why the Correct Answer is Right
To calculate the incidence of myopia per 100, we first need to determine the population at risk at the beginning of the period, which is the total population minus those already affected (to avoid counting them as new cases). Initially, there were 500 cases of myopia, so the population at risk = 5000 - 500 = 4500. Over the year, 90 new cases were found. The incidence rate per 100 is calculated as (number of new cases / population at risk) * 100 = (90 / 4500) * 100 = 2.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an incorrect calculation or misunderstanding of the concept of incidence.
- **Option B:** Similarly, this option does not align with the correct calculation of incidence rate as described.
- **Option D:** This option also does not match the correct calculation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **incidence** reflects the rate of new disease occurrences, while **prevalence** reflects the total number of cases (new and existing) in a population at a given time. Understanding the difference is crucial for interpreting epidemiological data.

## Correct Answer Line
**Correct Answer: C. 2.**