Which of the following pairs of compounds are not positional isomers?

For the following Newman projection,identify the corresponding sawhorse projection.

The total number of isomers,considering both structural and stereoisomers,of cyclic ethers with the molecular formula $C_4H_8O$ is. . . . . .

Which of the following is an $E$ isomer?

Which of the following structures represents a meso-compound?

In the projection formula $[A]$,two groups ($Br$ and $CH_3$) at the chiral center are interchanged to obtain formula $[B]$. When two groups ($C_2H_5$ and $Cl$) of $[A]$ are interchanged,the projection formula $[C]$ is obtained. Which of the following statements is $NOT$ correct for $[A]$,$[B]$,and $[C]$?