Janessa loves to collect scientific objects. In her collection, she has a magnet and several iron objects that are used to demonstrate center of gravity. When Janessa places her collection on a shelf together, however, the magnet keeps attracting the iron objects, thus knocking them over. She wonders how far away the magnet needs to be placed from the objects so they will no longer fall down. To determine this distance, Janessa performs an experiment in which she fastens her magnet to a table. Then, she holds one of her iron objects 1 cm away from the magnet. She lets go of the object and observes if the object falls down. If the object falls down, she increases the distance an additional centimeter. She repeats these steps until the object no longer falls down once it is released.

What is the test variable (independent variable) in Janessa's experiment?

A. The type of magnet that was used
B. The force of attraction between the magnet and the object
C. The iron that made up the objects
D. The distance between the magnet and the object