As an ideal gas expands isothermally, its volume increases while its temperature is kept fixed. What does the source say must happen to heat transfer during this expansion, and why?

1. As a gas expands, it would ordinarily cool down unless energy is supplied to it. 2. To keep the temperature fixed during an isothermal expansion, heat transfer must occur from the surroundings into the gas. 3. This rules out option A, since keeping temperature constant is exactly why heat transfer is required, not a sign that none occurs. 4. Heat leaving the gas would cause further cooling on top of the cooling from expansion, which is the opposite of what keeps temperature fixed, ruling out option C. 5. Only one continuous inward flow throughout the expansion is described here, rather than an alternating pattern, 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_