The isomers which can be converted into another forms by rotation of the molecules around single bond are

Explain the change of energy with a graph for the internal rotation of ethane about the $C-C$ bond after a $120^{\circ}$ rotation.

In the Newman projection for $2,2$-dimethylbutane,what can $X$ and $Y$ respectively be?
$A$. $H$ and $H$
$B$. $H$ and $C_2H_5$
$C$. $C_2H_5$ and $H$
$D$. $CH_3$ and $CH_3$

Compare the stabilities of the following two compounds $(A)$ and $(B)$:
$A$ : $cis-1$-ethyl-$3$-methylcyclohexane
$B$ : $trans-1$-ethyl-$3$-methylcyclohexane

Which of the following angles in ethane remains constant,and which angle changes during rotation about the $C-C$ single bond?

Which is the most stable conformation of ethylene glycol?