Four vessels A, B, C and D contain respectively 20 g atom $\left({\mathrm{T}}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=5 \mathrm{h}\right)$, 2 g atom $\left({\mathrm{T}}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=1 \mathrm{h}\right)$ and 5 g atom $\left({\mathrm{T}}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=2 \mathrm{h}\right)$ and 10 g atom $\left({\mathrm{T}}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}=3 \mathrm{h}\right)$ of different radio nuclei in the beginning. The maximum activity would be exhibited by the vessel, is