The shells of some marine snails are highly variable, with some snail shells being solid colors and others having stripes. predation by birds is a major source of mortality for the snails. scientists studying these snails found that while 47 percent of the snail population was striped, 56 percent of the snails eaten by birds were striped. what could scientists conclude about the snails?

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?