Which of the following is distilled first during the fractional distillation of liquid air or mixtures of these gases?

In an ice crystal,each water molecule is hydrogen bonded to $.......$ neighbouring molecules.

Identify the correct statements from the following:
$I$. $B_2H_6$ is an electron deficient hydride.
$II$. $NH_3$ is an electron rich hydride.
$III$. $NaH$ is a covalent hydride.
$IV$. $YbH_{2.55}$ is an interstitial hydride.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Physical properties of isotopes of hydrogen are different.
Reason $R$: Mass difference between isotopes of hydrogen is very large.
In the light of the above statements,choose the correct answer from the options given below:

Calcium imide on hydrolysis gives gas $(B)$ which on oxidation by bleaching powder gives gas $(C)$. Gas $(C)$ on reaction with magnesium gives compound $(D)$ which on hydrolysis again gives gas $(B)$. Identify $(B)$,$(C)$ and $(D)$.

What do you understand by $(i)$ electron deficient,$(ii)$ electron precise,and $(iii)$ electron rich compounds of hydrogen? Provide justification with suitable examples.