Identify the group of the periodic table that contains the element $Astatine$ $(At)$.

When treated with conc. $HCl$,$MnO_2$ yields a gas $(X)$ which further reacts with $Ca(OH)_2$ to generate a white solid $(Y)$. The solid $Y$ reacts with dil. $HCl$ to produce the same gas $X$. The solid $Y$ is

The mixture of conc. $HCl$ and potassium chlorate on heating gives

Marine plants are an important source of ........ .

Oxygen does not react with

The correct order of acidic strength is