It is much easier for a small animal to run uphill than for a large animal, because:

Options

A. smaller animals have a higher metabolic rate
B. small animals have a lower `O_(2)` requirement.
C. the efficiency of muscles in large animals is less than in the small animals.
D. It is easier to carry a small body weight.

Answer

To solve the question "It is much easier for a small animal to run uphill than for a large animal, because:", we need to analyze the options provided and understand the physiological differences between small and large animals.

### Step-by-Step Solution:

1. **Understanding Metabolic Rate**:
- Metabolic rate refers to the rate at which an organism converts food into energy. Smaller animals generally have a higher metabolic rate compared to larger animals. This means they can generate energy more quickly.

**Hint**: Remember that metabolic rate is inversely proportional to body size. Smaller animals need to consume more energy relative to their size.

2. **Analyzing the First Option**:
- The first option states that "smaller animals have a higher metabolic rate." This is true. The higher metabolic rate allows small animals to perform activities, such as running uphill, more efficiently.

**Hint**: Consider how energy requirements change with size; smaller animals often need to expend more energy to maintain their body functions.

3. **Evaluating the Second Option**:
- The second option claims that "small animals have lower O2 requirement." This is incorrect. Smaller animals typically have a higher oxygen requirement relative to their body size due to their higher metabolic rates.

**Hint**: Think about how oxygen consumption is related to energy production in animals.

4. **Reviewing the Third Option**:
- The third option suggests that "the efficiency of muscles in large animals is less than in small animals." This is also incorrect. Larger animals generally have more muscle mass and can be more efficient in certain physical activities.

**Hint**: Efficiency in muscle function can vary, but larger animals often have advantages in strength due to their size.

5. **Considering the Fourth Option**:
- The fourth option states that "it is easier to carry small body weight." While this may seem plausible, it is too general and does not apply universally across all species. The ease of carrying weight can depend on various factors, including the animal's anatomy and environment.

**Hint**: Think about how different species adapt to their environments and how that affects their physical capabilities.

6. **Conclusion**:
- After evaluating all the options, the correct answer is that "smaller animals have a higher metabolic rate," which allows them to run uphill more easily than larger animals.

### Final Answer:
The correct answer is: **Smaller animals have a higher metabolic rate.**