Which of the following compounds of boron is the hardest?

Which gases are released when aluminum reacts with concentrated $HCl$ and concentrated $NaOH$,respectively?

The type of hybridisation of boron in diborane is

Diborane reacts with $HCl$ in the presence of $AlCl_3$ and liberates:

Which of the following statements about $BF_4^{-}$ and $AlF_6^{3-}$ are correct?
$(i)$ $B$ and $Al$ differ in their oxidation states.
$(ii)$ $B$ and $Al$ differ in their covalency.
$(iii)$ $B$ obeys the octet rule.
$(iv)$ $B$ and $Al$ are in diagonal relationship.

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