Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: The helium gas is more active in the winter season.
 
- B: Air outside the balloons leaks into the balloons.
 
- C: As the temperature increases, the helium in the balloons expands.
 
- D: Outdoor air pressure in the summer is less than indoor air pressure.
 