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【单选题】
纳米级四氧化三铁是应用最为广泛的软磁性材料之一。共沉淀法是目前制备纳米四氧化三铁的重要方法，其流程如图示： 下列说法不正确的是A.
保存溶液A时，应加入少量
B.
其制备反应类型不属于氧化还原反应
C.
取滤液进行焰色反应，火焰为黄色
D.
FeSO 4 ·7H 2 O 和 FeCl 3 ·6H 2 O的物质的量之比最好应为2∶1
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