Kidneys eliminate the waste materials in the liquid form called ______.

Explanation

**Step-by-Step Solution:**

1. **Identify the Function of Kidneys**: The kidneys are vital organs in the human body that perform the function of excretion. They filter blood to remove waste products.

2. **Understand What Waste is Produced**: One of the main waste products produced by the body is urea. This is a byproduct of protein metabolism.

3. **Determine the Form of Excretion**: The kidneys eliminate waste materials in a specific form. In this case, the waste material (urea) is eliminated in a liquid state.

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