In the structure of borax,the numbers of boron atoms and $B-O-B$ units,respectively,are

The hardest substance amongst the following is

Which of the following is $NOT$ true regarding the structure of diborane $(B_2H_6)$?

Sodium borate on reaction with $H_2SO_4$ (conc.) and $C_2H_5OH$ gives a compound '$A$' which burns with a green flame. The compound '$A$' is

When $Al$ is added to $KOH$ solution,what happens?

Boron trifluoride on reaction with lithium aluminium hydride in ether gives $LiF$, $AlF_3$ and $X$. $X$ on reaction with $NH_3$ gives $Y$. $Y$ on further heating gives a compound $Z$. The number of $\sigma$-bonds and $\pi$-bonds in $Z$ are $x$ and $y$ respectively. $(x+y)$ is equal to