单选题
When the
concentration
of calcium in the blood is
too
low, the parathyroid glands
began
to
secrete
the hormone parathormone.
A、
concentration
B、
too
C、
began
D、
secrete
【正确答案】
C
【答案解析】
[解析] began改为begin。主、从句的时态保持一致。
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