_____________ are able to maintain homeostasis which ensures a constant internal environment of the body.

Explanation

**Step-by-Step Solution:**

1. **Understanding Homeostasis**: Begin by defining homeostasis. Homeostasis refers to the ability of an organism to maintain a stable internal environment despite changes in external conditions. This includes regulating factors such as temperature, pH, hydration, and nutrient levels.

2. **Identifying Organisms**: Next, identify which organisms are capable of maintaining homeostasis. This typically includes animals that have complex physiological processes.

3. **Classifying Organisms**: Among animals, certain groups are particularly noted for their ability to regulate their internal environment. Birds and mammals are prime examples of such organisms.

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