In veins, valves are present to check backward flow of blood flowing at

Options

A. atmospheric pressure
B. high pressure
C. low pressure
D. all of these

Explanation

To solve the question regarding the presence of valves in veins and their function in preventing the backward flow of blood, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Function of Veins**:
- Veins are blood vessels that carry deoxygenated blood from the body tissues back to the heart. This blood flow is essential for returning blood to the heart for re-oxygenation.

2. **Identify the Nature of Blood Flow in Veins**:
- The blood in veins flows at a relatively slow rate compared to arteries. This slow flow is due to the lower pressure in veins.

3. **Recognize the Problem of Backward Flow**:
- Because the blood is moving slowly, there is a higher risk of it flowing backward due to gravity and other factors.

4. **Role of Valves in Veins**:
- Valves in veins are crucial as they prevent the backward flow of blood. They open to allow blood to flow toward the heart and close to prevent it from flowing back.

5. **Evaluate the Options Provided**:
- The options given are:
1. Atmospheric pressure
2. High pressure
3. Low pressure
4. All of these
- Since the blood in veins flows at low pressure, the presence of valves is particularly important in this context.

6. **Select the Correct Answer**:
- The correct answer is **low pressure** (Option 3), as this is the condition under which the valves function to prevent backward flow.

### Final Answer:
The correct answer is **low pressure**.

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