Rate of breathing is controlled mainly by

Options

A. `CO_(2)` is level in blood
B. pH in blood
C. `O_(2)` level in blood
D. `O_(2)` level and pH in blood.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks what primarily controls the rate of breathing. This involves understanding the physiological mechanisms behind breathing regulation.

2. **Identifying the Options**: The options provided are:
- A) Carbon dioxide level in blood
- B) pH in blood
- C) Oxygen level in blood
- D) Oxygen level and pH in blood

3. **Analyzing Each Option**:
- **Option A (Carbon dioxide level in blood)**: Carbon dioxide (CO2) is a key factor in regulating breathing. An increase in CO2 levels leads to a decrease in pH (making the blood more acidic), which stimulates breathing.
- **Option B (pH in blood)**: While pH is influenced by CO2 levels, it is not the primary control mechanism. It is a result of CO2 concentration changes.
- **Option C (Oxygen level in blood)**: Oxygen levels do play a role in breathing, but they are not the primary regulator. The body is more sensitive to changes in CO2 than to changes in oxygen.
- **Option D (Oxygen level and pH in blood)**: This combines two factors, but again, CO2 is the main driver.

4. **Conclusion**: The primary control of the rate of breathing is mainly due to the concentration of carbon dioxide in the blood. When CO2 levels rise, it leads to increased breathing rates to expel excess CO2.

5. **Final Answer**: Therefore, the correct answer is **A) Carbon dioxide level in blood**.

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