Total number of isomers, considering both structural and stereoisomers, of cyclic ethers with the molecular formula ${\mathrm{C}}_4{\mathrm{H}}_8\mathrm{O}$ is ________

For the given compound X, the total number of optically active stereoisomers is ____.

What is the order of basicity among the following compounds?

Newman projections $\mathrm{P},\mathrm{Q},\mathrm{R}$ and $\mathrm{S}$ are shown below:

  Which one of the following options represent identical molecules?

The Fischer projection of $\mathrm{D}$-erythrose is shown below.

$\mathrm{D}$-Erythrose and its isomers are listed as $\mathrm{P},\mathrm{Q},\mathrm{R},$ and $\mathrm{S}$ in Column I. Choose the correct relationship of $\mathrm{P},\mathrm{Q},\mathrm{R},$ and $\mathrm{S}$ with $\mathrm{D}$-erythrose from Column II.

	Column - I		Column -II
P.		1.	Diastereomer
Q.		2.	Identical
R.		3.	Enantiomer
S.

With respect to the compounds $\mathrm{I}-\mathrm{V}$ choose the correct statement(s).