A gas expands adiabatically, meaning no heat transfer occurs during the process. According to the source, what happens to the gas's temperature during this adiabatic expansion, and why?

1. In an adiabatic process, Q = 0 by definition, so the first law reduces to ΔU = -W. 2. During an expansion, the gas does positive work W on its surroundings, which makes ΔU negative. 3. A negative ΔU corresponds to a drop in internal energy, and for a gas that drop shows up as a lower temperature. 4. Option A confuses zero heat transfer with zero temperature change; instead, it is precisely the absence of heat transfer that forces the temperature to fall as work is done. 5. Option B has the sign of the work backward, since the gas is expanding and doing work outward rather than having work done on it. 6. This temperature drop is stated as a general feature of adiabatic expansion, not one restricted to monatomic gases, ruling out option D. _Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.2 The First Law of Thermodynamics and Some Simple Processes_