As food moves through the digestive tract, it spends the most time in the:

a) esophagus because of the slow movement of peristalsis on chewed food.
b) stomach because of the extensive breakdown of carbohydrates and fats.
c) small intestine because of the time it takes to absorb through the villi.
d) large intestine because of the need to partially digest fiber and absorb water.

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?