In the following sets of reactants which two sets best exhibit the amphoteric characters of $Al_2O_3. xH_2O$ ?

Set $1$: $Al_2O_3 .xH_2O\, (s)$ and $OH^-(aq)$

Set $2$: $Al_2O_3 .xH_2O\, (s)$ and $H_2O\,(l)$

Set $3$: $Al_2O_3 .xH_2O\, (s)$ and $H^+(aq)$

Set $4$: $Al_2O_3 .xH_2O\, (s)$ and $NH_3(aq)$

Consider the following statements for diborane 

$1.$  Boron is approximately $sp^3$ hybridized

$2.$  $B-H-B$ angle is $180^o$

$3.$  There are two terminal $B-H$ bonds for each boron atom

$4.$  There are only $12$ bonding electrons available

Of these statements

Which of following salt does not give borax bead test 

Give uses of $LiBH_4$ and $NaBH_4$.

Which of the following is formed when aluminium oxide and carbon is strongly heated in dry chlorine gas

In the metallurgy of aluminium, cryolite is mixed in the molten state because it