单项选择题

In the calibration of Na2S2O3 solution, K2Cr2O7 is not used to titrate directly, but the indirect iodine content method is needed for the reason that (    )

A.There is no definite measurement of the reaction
B.There is no proper indicator
C.Reaction speed is too slow
D.K2Cr2O7 is not oxidizing enough
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