A Colorless solid `A` liberates a brown gas `B` on acidification, a colorless alkaline gas `(C)`on

A Colorless solid `A` liberates a brown gas `B` on acidification, a colorless alkaline gas `(C)`on treatment with `NaOH` and a colorless non-reactive gas `D` on heating. If heating of the solid is continued, it completely disappears. The compound `A` is

`(C)`on