Which is a secondary standard substance?

$\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{O}_{2} 2 \mathrm{H}_{2} \mathrm{O}$

$\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$

$\mathrm{KMnO}_{4}$

$\text { dried } \mathrm{Na}_{2} \mathrm{CO}_{3}$