Which of the following does not exist in free form?

$A$ and $B$ are formed when borax is dissolved in water. $C$ and $B$ are formed when borax is reacted with aqueous $HCl$ solution. What are $A$ and $C$ respectively?

Which of the following compounds exhibits acidic nature?

What is the correct formula of Borazole?

White fumes appear around the bottle of anhydrous aluminium chloride. Give reason.

Explain the atomic radius of elements of the boron family.