# The first and second dissociation constants of an acid $H_2A$ are $1.0 \times 10^{-5}$ and $5.0 \times 10^{-10}$ respectively. The overall dissociation constant of the acid will be :
( A ) $5.0 \times 10^{-5}$
( B ) $0.2 \times 10^{5}$
( C ) $5.0 \times 10^{15}$
( D ) $5.0 \times 10^{-15}$