In humans, the trachea is strengthened by incomplete ................... rings.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the type of rings that strengthen the trachea in humans. The trachea is also known as the windpipe and is a crucial part of the respiratory system.

2. **Identifying the Structure**: The trachea is a tube that conducts air to and from the lungs. It needs to maintain its structure to prevent collapse, especially when there is no air inside it.

3. **Function of the Rings**: The trachea is supported by rings that provide structural integrity. These rings are crucial for keeping the airway open, allowing for the passage of air during breathing.

...