All of following structure of the human respiratory system contains incomplete cartilaginous rings , except :

Options

A. Secondary bronchi and tertiary bronchi.
B. Trachea and primary bronchi
C. Initial bronchioles and tertiary bronchi
D. Distal bronchioles and terminal bronchioles

Answer

To solve the question, "All of the following structures of the human respiratory system contain incomplete cartilaginous rings, except:", we need to analyze each option provided and identify which structure does not contain incomplete cartilaginous rings.

### Step-by-Step Solution:

1. **Understanding the Structures**:
- The human respiratory system includes various structures such as the trachea, bronchi (primary, secondary, tertiary), and bronchioles (initial, distal, terminal).
- Incomplete cartilaginous rings are primarily found in the trachea and bronchi.

2. **Analyzing the Options**:
- **Option 1: Secondary Bronchi and Tertiary Bronchi**:
- Both secondary and tertiary bronchi contain incomplete cartilaginous rings. Therefore, this option is incorrect.

- **Option 2: Trachea and Primary Bronchi**:
- The trachea (windpipe) has incomplete cartilaginous rings, and primary bronchi also have these rings. Thus, this option is also incorrect.

- **Option 3: Initial Bronchioles and Tertiary Bronchioles**:
- Initial bronchioles do not have cartilaginous rings; they are primarily made up of smooth muscle. Tertiary bronchioles, however, still have some cartilage. This option is misleading as it combines structures with different characteristics.

- **Option 4: Distal Bronchioles and Terminal Bronchioles**:
- Both distal and terminal bronchioles do not contain cartilaginous rings. They are primarily composed of smooth muscle and are the final branches of the bronchi. Therefore, this option is correct.

3. **Conclusion**:
- The structure that does not contain incomplete cartilaginous rings is **Option 4: Distal Bronchioles and Terminal Bronchioles**.

### Final Answer:
The correct answer is **Option 4: Distal Bronchioles and Terminal Bronchioles**.