单项选择题

络合滴定中，金属指示剂的封闭是因为
A、金属离子或杂质离子与指示剂的稳定性太高
B、指示剂被氧化
C、金属离子不与EDTA作用
D、金属离子不与指示剂作用

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Negative feedback is usually adopted to stabilize the output voltage of a linear regulator.

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identify the rhetorical devices of the underlined part in the following sentences:
3. From them all Mark Twain gained a keen perception of the human race, of the difference between what people claim to be and what they really are. A.
antithesis B.
metaphor C.
pun D.
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