`KMnO_(4)` is not used as primary standard because
A. It is easily obtained in pure state free from `MnO_(2)`
B. It is very difficulty to obtain in pure state as it is not free from `MnO_(2)`.
C. It is very difficulty in pure state free from `MnO_(2)`
D. It is easily obtained in pure state but not free from` SO_(2)`.