Between which two adjacent distances did the largest decrease in the size of the pupil take place?

The question above is about an experiment that was performed on eyes.
The experiment involves covering the left eyes of the experimental subject while the right eye is exposed to light at different distances and the correspondent size of the pupil measured.
In the graph given for the experiment, the largest decrease in the size of the pupil take place between 7 and 8 distances. At distance 7, the size of the pupil is 4mm while at distance 8, the size is 1mm, which means that, there is a decrease of 3mm in pupil size. This is the largest decrease on the graph.

The answer is one amu.