Gary has a piece of diamond and a piece of graphite. he wants to test the hardness of these two minerals. when he rubs the diamond against the piece of graphite, the graphite gets scratched. when he rubs the piece of graphite against the diamond, the diamond does not get scratched. which of these is true? a. the graphite is harder than the diamond. b. this test does not show which mineral is harder. c. the diamond is harder than the graphite. d. both minerals have the same hardness.

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?