Answer:
C. The weight of each filled balloon
Explanation:
If the two balloons are the same and made of the same material, the student can observe the weight of each filled balloon to determine if the two balloons have the same amount of gas.
For this, he must have access to a precision scale, to position an empty balloon, to know what weight a balloon has when it is empty. Then, he must position each balloon filled with gas at a time, look at the weight indicated by the scale and subtract the weight of the empty balloon. If after that, the balloons show the same result, it means that the balloons have the same amount of the same gas.
