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?

a) By blocking on shaving brush size

b) By blocking on shaving brush type

c) By blocking on shaving brush shape

d) By blocking on lather thickness

e) Without blocking