The buildings in this old mine are made of metal. You can see over 100 years, the silver color of the buildings has changed. The red-brown color is rust. What does this color change tell us about the metal? A) The metal is malleable, so it bends easily. B) The metal reacts in the presence of air and water. C) The dry, desert air has damaged the building over the years. D) The metal is a good conductor of heat and electricity.

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?