use the same size of paper. buy the same color of paper. fold the paper the same way. decorate the paper the same way.
I gave my students this multiple choice question: A class wanted to see which paper airplane design could fly the farthest. In order to have the most accurate results, every student should

Only one answer is correct among these 4 choices:
- use the same size of paper.
- buy the same color of paper.
- fold the paper the same way.
- decorate the paper the same way.

Could you tell me which one is correct?

use the same size of paper.