The gas used for filling weather balloons is A. Helium B. Hydrogen C. Air D. Nitrogen

The correct answer is A. Helium.

Helium is a gas that is lighter than air. This means that it will float in the air. Weather balloons are filled with helium so that they can float up into the atmosphere and collect data about the weather.

Hydrogen is another gas that is lighter than air. However, hydrogen is highly flammable and can be dangerous to use. Air is a mixture of gases, including nitrogen, oxygen, and argon. Air is not as light as helium, so it is not used to fill weather balloons. Nitrogen is a gas that is heavier than air. It is not used to fill weather balloons because it would cause the balloons to sink.