This bar graph shows the average annual erosion of cropland in Montana for every five years between 1982 and 2010. The lighter part of each bar represents wind erosion while the darker part represents sheet and rill erosion. Sheet erosion is the erosion of a thin layer of topsoil over an entire area, usually caused by the particle movement and impact of rainfall Rill erosion occurs when runoff water create small concentrated channels as it runs down a slope.

Astudy by solloch and et. al., in 2017, gives the map above which shows the frequency of alleles with a ccr5-delta32 mutations over 87 different countries. this mutation deletes the presence of a co-receptor (ccr5) on the outside of human t-cells (lymphocytes). some viruses, such as the one responsible for the black death and human immunodeficiency virus (hiv), require this receptor for attachment to host cells during the infection process. the black death was an epidemic that passed over northern europe during the 14th century killing nearly 60% of europeans. according to this information, which explanation best explains why northern europeans show a greater immunity for hiv than some other parts of the world?