Facial shaving brushes are either bulb-shaped or fan-shaped. An experiment is to be conducted to determine whether the lather thickness is the same for both types of shaving brushes. In previous studies, it was determined that shaving brush size (small, medium, large) is associated with lather thickness, but shaving brush type (Wolf Whiskers, Maggard Razors, Frank Shaving, and so on) is not associated with lather thickness. How could the experiment BEST be conducted?

By blocking on shaving brush size

By blocking on shaving brush type

By blocking on shaving brush shape

By blocking on lather thickness

Without blocking