Which one of the following statements is accurate?
a. electron microscopes are tools used to study living organisms when they're alive. b. false colors are added to images to make some structures more noticeable. c. electron microscopes focus beams of light using magnetic fields. d. compound light microscopes utilize lenses and beams of electrons to produce images.

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?