The number of terminal and bridging hydrogens in $B_2H_6$ are respectively

How would you explain the lower atomic radius of $Ga$ as compared to $Al$?

When orthoboric acid $(H_3BO_3)$ is heated,the residue left is:

What are the correct statements about the elements of group $13$ given below?
$(I)$ The stability of $+1$ oxidation state follows the order $Tl > In > Ga$
$(II)$ Boron has the lowest melting point and boiling point as it is a non-metal
$(III)$ Boron shows a maximum covalency of $4$ in its compounds
$(IV)$ The order of atomic radius is $Ga > Al > In$
$(V)$ Aluminium is passive to concentrated nitric acid

The correct statements among $I$ to $III$ regarding group $13$ element oxides are,
$I$. Boron trioxide is acidic
$II$. Oxides of aluminium and gallium are amphoteric
$III$. Oxides of indium and thallium are basic

Group-$13$ elements react with $O_2$ to form oxides of the type $M_2O_3$ ($M =$ element). Which among the following is the most basic oxide?