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.

• A
  $U_{1}$ increases and $U_{2}$ decreases and the total entropy remains the same
• B
  $U_{1}$ decreases and $U_{2}$ increases and the total entropy remains the same
• C
  $U_{1}$ increases and $U_{2}$ decreases and the total entropy increases
• D
  $U_{1}$ decreases and $U_{2}$ increases and the total entropy increases