The ears work with the brain and other body parts through a series of steps that help you hear the sound of a train coming. What step must happen before your brain recognizes that this is the sound of a train moving? Your brain sends a message through the nerves to the muscles in your legs.
Your ears send a message through the nerves to the brain.
Your leg muscles pull on the bones as you move out of the way.
Your eyes pick up the sound waves as you see the train coming.

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?