In which of the following groups of animals the lungs are relatively non-elastic and are connected with elastic airsacs?

Options

A. Mammals
B. Birds
C. Reptiles
D. Amphibians

Explanation

To determine which group of animals has relatively non-elastic lungs connected with elastic air sacs, we can follow these steps:

### Step 1: Understand the Structure of Lungs in Different Animals
- **Mammals**: Mammalian lungs are elastic and change volume during breathing, which creates a pressure difference that facilitates inhalation and exhalation.
- **Birds**: Bird lungs are relatively non-elastic and are connected to elastic air sacs. This unique structure allows for a continuous flow of air through the lungs, enhancing the efficiency of gas exchange.

### Step 2: Identify the Role of Air Sacs
- In birds, the air sacs play a crucial role in respiration. They allow for a unidirectional flow of air, which means that fresh air is always available for gas exchange, even during exhalation. This adaptation is particularly important for birds, especially during flight when they require a high oxygen intake.

### Step 3: Compare with Other Animal Groups
- Other groups like reptiles, amphibians, and fish do not have the same structure as birds. For example:
- **Reptiles**: Have lungs that are more elastic but do not have air sacs.
- **Amphibians**: Use skin and lungs for respiration, but their lung structure is also different from that of birds.
- **Fish**: Use gills for respiration, which is entirely different from lung-based respiration.

### Step 4: Conclusion
- Based on the characteristics of the respiratory systems, we conclude that the group of animals with relatively non-elastic lungs connected with elastic air sacs is **birds**.

### Final Answer
- The correct answer is **birds**.

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