The lymph serves to

Options

A. return the interstitial fluid to the blood
B. return he WBCs and RBCs to the lymph nodes
C. transport `CO_(2)` to the lungs
D. transport `O_(2)` to the brain.

Explanation

### Step-by-Step Solution:

1. **Understanding Lymph**: Lymph is a fluid that is formed from interstitial fluid, which is the fluid that surrounds cells in tissues. It plays a crucial role in the body's immune system and fluid balance.

2. **Formation of Lymph**: Interstitial fluid is collected from the spaces between cells and is drained into lymphatic vessels. This process helps in maintaining the fluid balance in the body.

3. **Function of Lymph**: The primary function of lymph is to return the interstitial fluid back to the bloodstream. This helps in preventing the accumulation of excess fluid in tissues, which can lead to swelling (edema).

4. **Evaluating the Options**:
- **Option 1**: "Return the interstitial fluid to the blood" - This is correct as it describes the main function of lymph.
- **Option 2**: "Return the WBC and RBC to the lymph nodes" - This is incorrect because while lymph does transport white blood cells (WBCs), it does not return red blood cells (RBCs) to the lymph nodes.
- **Option 3**: "Transport CO2 to the lungs" - This is incorrect as CO2 is primarily transported by the blood, not lymph.
- **Option 4**: "Transport O2 to the brain" - This is also incorrect since oxygen is carried by red blood cells in the bloodstream, not by lymph.

5. **Conclusion**: After evaluating all the options, the correct answer is that lymph serves to return interstitial fluid to the blood.

### Final Answer:
The lymph serves to return the interstitial fluid to the blood.

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