The respiratory medium flows unidirectionally over the respiratory surface in the gas-exchange systems of:

Options

A. mammals
B. frogs
C. birds
D. insects

Explanation

To answer the question regarding which organisms have a unidirectional flow of respiratory medium over their respiratory surfaces, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Respiratory Medium**: The respiratory medium refers to the medium through which gas exchange occurs, typically air or water.

2. **Identify Organisms with Respiratory Systems**: Different organisms have different respiratory systems. The main types include mammals, birds, amphibians, and insects.

3. **Analyze Mammals**: In mammals (like humans), the lungs facilitate gas exchange, but the air flow is bidirectional. Air is inhaled into the lungs and exhaled out, meaning it does not flow in one direction.

4. **Examine Amphibians**: Amphibians, such as frogs, also have lungs similar to mammals, and they too exhibit bidirectional flow of air during respiration.

5. **Look at Insects**: Insects do not have lungs; instead, they have a tracheal system that allows for gas exchange directly with their tissues. The flow of air is not unidirectional in a way that is comparable to lungs.

6. **Focus on Birds**: Birds have a unique respiratory system that includes lungs and air sacs. The air sacs allow for a unidirectional flow of air through the lungs. This means that air moves in one direction, providing a continuous supply of fresh air for gas exchange, which is particularly efficient during flight.

7. **Conclusion**: Based on the analysis, the correct answer to the question is that the respiratory medium flows unidirectionally over the respiratory surface in the gas-exchange systems of **birds**.

### Final Answer:
The respiratory medium flows unidirectionally over the respiratory surface in the gas-exchange systems of **birds**.

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