The element which does not belong to group $15$ is

An amorphous solid $'A'$ burns in air to form a gas $'B'$ which turns lime water milky. The gas is also produced as a by-product during roasting of sulphide ore. This gas decolourises acidified aqueous $KMnO_4$ solution and reduces $Fe^{3+}$ to $Fe^{2+}$. Identify the solid $'A'$ and the gas $'B'$ and write the reactions involved.

Select the correct statement.

What is the structure of the dithionate ion?

The colour of liquid $O_2$ is

Sulphur trioxide can be obtained by which of the following reactions?