Q 1 :    

Which of the following species contains equal number of $\sigma$ and $\pi$-bonds?                 [2015]
 

• ${\left(\mathrm{CN}\right)}_2$

   

• ${\mathrm{CH}}_2{\left(\mathrm{CN}\right)}_2$

   

• ${\mathrm{HCO}}_3^{-}$

   

• ${\mathrm{XeO}}_4$