When the A(concentration) of calcium in the blood is B(too) low, the parathyroid

游客2025-05-07 1

问题 When the A(concentration) of calcium in the blood is B(too) low, the parathyroid glands C(began) to D(secrete) the hormone parathormone.

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答案 C

解析 began to secrete改为begin。主从句的时态保持一致。

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