A rigid, thermally insulated vessel is separated into two equal-volume compartments by a fixed, thin, thermally conducting partition. Section 1 contains \(n\) moles of an ideal monatomic gas at an initial temperature \(2T_0\) and initial internal energy \(U_{1,i}\). Section 2 contains \(3n\) moles of the same ideal monatomic gas at an initial temperature \(T_0\). The system is allowed to reach thermal equilibrium. Which of the following correctly identifies the change in internal energy \(\Delta U_1\) of the gas in Section 1 and provides a valid microscopic justification for the resulting pressure change in Section 1?