Which of the following element$(s)$ exist in the liquid state?

Identify the compounds $A$,$X$ and $Z$ in the following reactions :
$A + 2HCl + 5H_2O \to 2NaCl + X$
$X$ $\xrightarrow[370 \ K]{\Delta} HBO_2$ $\xrightarrow[> 370 \ K]{\Delta} Z$

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:

Boron is unable to form $BF_{6}^{3-}$ ion. Explain.

Which one of the following is correct relating to diborane $(B_2H_6)$?

Marine plants are an important source of which of the following?