What is an example of external control that maintains an ethnic groups' identity?

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

Giving free brainliest + free points if answered correctly ! rural areas have of development. a. heavy signs b. no signs c. one type d. different types

Why does the blending of many different diciplines present both challenges and opportunities to enviromental scientist?