Here's a problem to solve: Air has no color and cannot be seen, yet it takes up space. What could be done to show that air takes up space?

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

- A: observe clouds forming
 
- B: measure the air temperature
 
- C: blow up a beach ball or balloon
 
- D: weigh a glass before and after it is filled with water
 