Compounds that should not be used as primary standards in titrimetric analysis are :
A. $\mathrm{Na}_2 \mathrm{Cr}_2 \mathrm{O}_7$
B. Oxalic acid
C. NaOH
D. $\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2 \mathrm{O}$
E. Sodium tetraborate
Choose the most appropriate answer from the options given below: