What is the frequency of an event? this is for forensic science

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

The biome pictured above is found in areas such as kenya and south africa. the land is grass-covered with very few trees and gazelles and other grazers are common here. why does this biome have a low population of trees? a) large grazing animals destroy the trees. b) it is a deforested rainforest with poor soil. c) it has high temperatures and low precipitation. d) it has a long rainy season followed by flooding.

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