Most stable conformation of $n$-butane is

Which of the following conformations will be the most stable?

In the following Newman projection of the skew conformation of ethane,the dihedral angle between $H'$ and $H''$ is $......^\circ$. (in $^\circ$)

The two structures $I$ and $II$ represent:

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

The dihedral angle in the staggered form of the Newman projection of $1,1,1-$trichloroethane is $60^{\circ}$. (Round off to the nearest integer)