When plasma Ca concentration decreases... 1. Parathyroid hormone level increases 2. Parathyroid hormone level decreases 3. Ca and phosphate reabsorption increases 4. Ca and phosphate reabsorption decreases 5. Ca reabsorption decreases and phosphate reabsorption increases 6. Ca reabsorption increases and phosphate reabsorption decreases

2. Parathyroid hormone level decreases

4. Ca and phosphate reabsorption decreases

Explanation:

When plasma Ca concentration decreases, the action of Parathyroid hormone level decreases  which leads to Calcium and phosphate reabsorption decreases. Parathyroid hormone is responsible for the increase of calcium and phosphorus in the blood when there is low level in the blood so when the re is huge amount of calcium and phosphorus in the blood, the Parathyroid hormone activity decreases which decrease the reabsorption of calcium and phosphorus decreases.

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here is your answer hope i .

im not intirely sure but i think the anwser is d relative humidity