If an ideal gas is perfectly insulated from its surroundings (so that no heat may flow), and the gas expands (the volume of the gas increases),

1. the temperature of the gas must increase.
2. the temperature of the gas must decrease.
3. the temperature of the gas must remain the same.
4. we would need more information to make a statement about the temperature.

(Hint: If you know what happens to the internal energy, you know what happens to the temperature.)