Which species behave as Brønsted-Lowry acids in the following reversible reaction?

$${{\text{H}}_2}{\text{PO}}_4^ - ({\text{aq)}} + {\text{C}}{{\text{N}}^ - }({\text{aq)}} \rightleftharpoons {\text{HCN(aq)}} + {\text{HPO}}_4^{2 - }({\text{aq)}}$$

A.     HCN and ${\text{C}}{{\text{N}}^ - }$

B.     HCN and ${\text{HPO}}_4^{2 - }$

C.     ${{\text{H}}_{\text{2}}}{\text{PO}}_4^ -$ and ${\text{HPO}}_4^{2 - }$

D.     HCN and ${{\text{H}}_{\text{2}}}{\text{PO}}_4^ -$

D