A group of students is investigating how the addition of salt impacts the floatation of an egg in water. Four identical cups were taken for the experiment. The data of the experiment are shown.

Experimental Record
Cup Amount of Water (mL) Amount of Salt (g) Level of Egg above Water
1 450 0 Sinks to bottom
2 450 68 2 cm
3 450 136 3 cm
4 450 272 4 cm

What was the independent variable in this experiment?
a Level of egg above water
b Amount of water in the cup
c Amount of salt added
d Type of cup