Ammonia is combined with carbon dioxide to form urea in kidney.

Explanation

### Step-by-Step Solution:

1. **Understanding the Reaction**: The question states that ammonia (NH3) combines with carbon dioxide (CO2) to form urea (NH2CO(NH2)). This is a biochemical reaction that occurs in the body.

2. **Identifying the Correct Process**: The process of converting ammonia and carbon dioxide into urea is known as the urea cycle. This reaction is crucial for detoxifying ammonia, which is toxic at high levels.

3. **Location of the Reaction**: The question mentions that this reaction occurs in the kidney. However, it is important to note that this reaction actually takes place in the liver, not the kidneys.

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