Assertion :- Thyrocalcitonin & Parathormone have antagonistic effect on blood calcium level.
Reason:- Thyrocalcitonin lower the blood calcium level and parathorrnone raises the blood calcium level by removal of calcium from bone and reabsorption of calcium from nephrons.

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 Thyrocalcitonin and Parathormone have antagonistic effects on blood calcium levels. This means that these two hormones work in opposite ways regarding the regulation of calcium in the blood.

2. **Identifying the Hormones**:
- **Thyrocalcitonin (Calcitonin)**: This hormone is produced by the thyroid gland. Its primary function is to lower blood calcium levels.
- **Parathyroid Hormone (PTH)**: This hormone is produced by the parathyroid glands. Its main role is to increase blood calcium levels.

3. **Explaining the Effects of Thyrocalcitonin**:
- Thyrocalcitonin lowers blood calcium levels by promoting the deposition of calcium in the bones and inhibiting the reabsorption of calcium in the kidneys. This results in decreased calcium levels in the bloodstream.

4. **Explaining the Effects of Parathyroid Hormone**:
- Parathyroid hormone increases blood calcium levels by stimulating the release of calcium from bones (bone resorption) and enhancing the reabsorption of calcium in the kidneys. It also increases the absorption of calcium from the diet in the intestines.

5. **Conclusion on the Assertion and Reason**:
- Both the assertion and reason are correct. The assertion correctly states that Thyrocalcitonin and Parathormone have opposing effects on blood calcium levels, while the reason accurately describes how each hormone functions to either lower or raise calcium levels in the blood.

6. **Final Answer**: Therefore, the correct option is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.