Which of the following is a free-living nitrogen-fixing bacterium?

Statement $P$: In vertebrates,about $70$ percent of the total phosphorus is incorporated into bones and teeth.
Statement $Q$: Phosphorus is a structural component of vitamins like biotin and thiamine.

Match Column-$I$ and Column-$II$ and choose the correct option given below the columns.

Column-$I$ (Element)	Column-$II$ (Function)
$A$. Calcium	$I$. Required for ionic balance
$B$. Boron	$II$. Essential for constitution of nucleic acid
$C$. Phosphorus	$III$. Required for absorption of calcium
$D$. Chlorine	$IV$. Required to activate respiratory enzyme
$E$. Manganese	$V$. Required for synthesis of mitotic spindle

Phosphorus is required for making
$I.$ shell
$II.$ bones
$III.$ teeth
Choose the correct option

Select the correct match for $X$ and $Y$.

Column $X$	Column $Y$
$(1)$ Magnesium	$(P)$ Blood clotting
$(2)$ Molybdenum	$(Q)$ Constituent of cysteine amino acid
$(3)$ Calcium	$(R)$ Essential for nitrogen fixation
$(4)$ Sulphur	$(S)$ Activation of enzymes

Nitrogen is required mainly by which of the following parts of the plants?
$I.$ Meristematic tissues
$II.$ Differentiating tissues
$III.$ Apical tissues
$IV.$ Metabolically active cells
Choose the correct option.