State the Kelvin-Planck statement of the second law of thermodynamics.

$Assertion :$ When a glass of hot milk is placed in a room and allowed to cool,its entropy decreases.
$Reason :$ Allowing a hot object to cool does not violate the second law of thermodynamics.

The graphs below show the entropy versus energy $U$ of two systems $1$ and $2$ at constant volume. The initial energies of the systems are indicated by $U_{1, i}$ and $U_{2, i}$,respectively. The graphs are drawn to the same scale. The systems are then brought into thermal contact with each other. Assume that,at all times,the combined energy of the two systems remains constant. Choose the most appropriate option indicating the energies of the two systems and the total entropy after they achieve equilibrium.

Why can't the efficiency of a heat engine ever be $100 \%$?

State the main points of the second law of thermodynamics.

State the Clausius statement of the second law of thermodynamics.