Which gas is used in hot air balloons? A. helium B. propane C. carbon dioxide D. nitrogen

The correct answer is A. Helium.

Helium is the gas used in hot air balloons because it is lighter than air. This means that when hot air is blown into a balloon, the balloon will rise because the hot air is less dense than the surrounding air. Helium is also non-flammable, which makes it safe to use in hot air balloons.

Propane is a flammable gas that is used in cooking and heating. Carbon dioxide is a gas that is used in fire extinguishers and carbonated beverages. Nitrogen is a gas that is found in the air. It is not lighter than air, so it would not be used in hot air balloons.