Assertion :- Parathormone & thyrocalcitonin are antagonist to each other.
Reason:- Parathormone maintains `Ca^(+2)` concen tration in blood and it's receptors are present in osteoclast cells.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that parathormone (PTH) and thyrocalcitonin (calcitonin) are antagonists to each other. This means that they have opposing effects on calcium levels in the blood.

2. **Identifying the Source of Hormones**:
- Parathormone is secreted by the parathyroid glands.
- Thyrocalcitonin is secreted by the thyroid gland.

3. **Function of Parathormone**:
- Parathormone increases the concentration of calcium ions (Ca²⁺) in the blood. It does this by stimulating osteoclasts, which are cells that break down bone tissue and release calcium into the bloodstream.

4. **Function of Thyrocalcitonin**:
- In contrast, thyrocalcitonin decreases the concentration of calcium ions in the blood. It does this by inhibiting osteoclast activity, thus reducing the release of calcium from bones.

5. **Analyzing the Reason**: The reason states that parathormone maintains calcium ion concentration in blood and that its receptors are present in osteoclast cells. This is true, as parathormone does act on osteoclasts to increase blood calcium levels.

6. **Conclusion on Assertion and Reason**: Both the assertion and reason are correct. However, the reason does not adequately explain why they are antagonistic to each other. The assertion correctly identifies their opposing roles, while the reason provides information about the function of parathormone but does not explain the relationship between the two hormones.

7. **Final Answer**: Both assertion and reason are correct, but the reason is not the correct explanation of the assertion.