You observe your friend rub a balloon on their wool sweater and then stick it to the wall. To investigate, you set up an experiment to test whether other objects will respond in the same way when you rub them against a sweater. What is the test variable in your experiment?

The number of times you rubbed the object on the sweater
The observation that the object sticks to the wall
The sweater you use to rub against the objects
The type of objects you are testing in the experiment