A group of students is investigating whether air resistance depends on the size of an object. The students throw two paper pieces each with the same force. One piece of paper is flat and the other is scrunched into a ball. The distance traveled by each paper piece is shown. Experimental Observations
Student 1 Student 2 Student 3 Student 4
Distance traveled by flat paper 0.2 m 1.2 m 1.5 m 0.8 m
Distance traveled by scrunched paper 1.5 m 2.4 m 3 m 2 m

What was the test variable (independent variable) in this experiment?
Distance traveled by flat paper
The shape of the paper
Distance traveled by scrunched paper
The number of students