A party shop delivers helium-filled balloons to homes and businesses. The owners realize from experience that on hot summer days they should inflate the balloons only three-quarters full. On cold winter days they can fully inflate the balloons. Which of the following is the BEST hypothesis to explain this observation? Air outside the balloons leaks into the balloons. The helium gas is more active in the winter season. As the temperature increases, the helium in the balloons expands. Outdoor air pressure in the summer is less than indoor air pressure.