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# Which of following is a secondary standard

$\begin{array}{1 1}(a)\;\text{Sodium hydroxide}\\(b)\;\text{Ferrous ammonium sulphate}\\(c)\;\text{Oxalic acid}\\(d)\;\text{Pottassium dichromate}\end{array}$

Secondary standard substances are those whose standard solution cannot be obtained by weight.$NaOH$ absorbs $CO_2$ and moisture from air.
Hence (a) is the correct answer.