Im not sure but if you think of it, the one that you didn't put any salt is a independent variable because it doesn't really rely on anything it doesn't have salt. The Dependent variable would be the 2nd and the 3rd cup because it doesn't have much salt but the 4th cup has lots of salt so its basically controlled. Hope it helps !
A population....... I think
Answer:
1. Circulatory System
2. White Blood Cells
3. Capillaries
4. Vein
5. Carbon Dioxide (CO²)
6. Circulatory System
7. Immune System , Fights
8. Red Blood Cells
9. Gas Exchange
10. Transport
11. Lungs
12. Artery, Heart and Lungs
13. Heart
14. Important Plant
15. Alveoli