Question: Which of the following mechanisms contributes to a decrease in plasma calcium concentration?

A: The action of vitamin D on the intestine
B: The action of parathyroid hormone on the intestine
C: The action of parathyroid hormone on the kidney
D: The action of calcitonin on the kidney
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option. Your entire response should be in English, and in all capital letters.
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