Aqueous solutions of ${\mathrm{HNO}}_3,\mathrm{KOH},{\mathrm{CH}}_3\mathrm{COOH}$ and ${\mathrm{CH}}_3\mathrm{COONa}$ of identical concentrations are provided. The pair (s) of solutions which form a buffer upon mixing is (are)

${\mathrm{HNO}}_3+{\mathrm{CH}}_3\mathrm{COONa}$ mixture can act as buffer solution if volume of ${\mathrm{HNO}}_3$ solution taken is lesser than volume of ${\mathrm{CH}}_3\mathrm{COONa}$ solution because of following reaction 

${\mathrm{CH}}_3\mathrm{COONa}+{\mathrm{HNO}}_3\to {\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{NaNO}}_3$ 

${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{CH}}_3\mathrm{COONa}$ mixture will act as buffer