Which of the following structures is similar to graphite?

The acid which forms two series of salts is

In the structure of diborane $(B_2H_6)$,the number of $2$-centre-$2$-electron $(2c-2e)$ bonds is $X$ and the number of $3$-centre-$2$-electron $(3c-2e)$ bonds is $Y$. The value of $(X+Y)$ is:

The structure of $Na_2[B_4O_5(OH)_4] \cdot 8H_2O$ contains:

Explain the physical and chemical properties of borax.

Discuss the pattern of variation in the oxidation states of $(i)$ $B$ to $Tl$ and $(ii)$ $C$ to $Pb$.