## **Core Concept**
A motor unit is defined as one motor neuron and all the muscle fibers it innervates. The number of muscle fibers in a motor unit varies greatly depending on the muscle's function and precision. Muscles requiring fine control, such as those in the eye, have smaller motor units.

## **Why the Correct Answer is Right**
The correct answer, **B. 10-20**, reflects the fact that muscles controlling eye movements have very small motor units. This is crucial for the precise control needed for eye movements. Having fewer muscle fibers per motor unit allows for more accurate and finer movements, which is essential for vision and tracking.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option suggests a very large number of muscle fibers per motor unit, which is more characteristic of muscles like the gastrocnemius that require a lot of power but not fine control.
* **Option C:** While closer, this range still does not accurately represent the number of muscle fibers in a motor unit of the eye muscles, which are known for their precision.
* **Option D:** This option suggests an even smaller number, but 1-5 might be too small to represent the typical range for eye muscles.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the size of a motor unit is inversely related to the precision of the muscle's control. Eye muscles have very small motor units (about 10-20 muscle fibers) for fine movements, whereas large muscles like the gastrocnemius have much larger motor units (hundreds to thousands of fibers) for powerful but less precise actions.

## **Correct Answer: B. 10-20**