From the following, select the feature that is common to all respiratory systems in animals.

Options

A. Active transport removes carbon dioxide from the respiratory structures .
B. Materials flow in one direction only .
C. An intake system is comprised of a series of tubes.
D. Gases diffuse across a moist membrane.

Answer

To solve the question regarding the common feature of all respiratory systems in animals, we can analyze the options provided and determine which feature is universally applicable.

### Step-by-Step Solution:

1. **Understanding the Options**:
- We need to evaluate each of the four statements to see if they apply to all animals.

2. **Evaluating Option 1**:
- **Active transport removes CO2 from the respiratory structure**: This is not common to all animals. Active transport is specific to certain organisms, particularly bacteria, and is not a universal feature in higher animals.

3. **Evaluating Option 2**:
- **Materials flow in one direction**: This is true for birds, which have a unique respiratory system allowing air to flow in one direction through their lungs. However, this is not a feature found in all animals.

4. **Evaluating Option 3**:
- **An intake system is comprised of a series of tubules**: This is characteristic of certain terrestrial organisms, particularly insects, which have a tracheal system. However, it does not apply to all animals.

5. **Evaluating Option 4**:
- **Moist Membrane**: This feature is found in all respiratory systems. Gas exchange occurs across moist membranes, such as in the alveoli of mammals, gills of fish, and even in the skin of amphibians. The presence of a moist surface is essential for the diffusion of gases in all respiratory systems.

6. **Conclusion**:
- The correct answer is **Moist Membrane**, as it is the only feature that is common to all respiratory systems in animals.

### Final Answer:
The feature that is common to all respiratory systems in animals is the **Moist Membrane**.