Why are the strands of dna said to be complementary to each other? a) the sugar-phosphate chains are running in opposite directions. b) the sugar-phosphate chains of each strand are identical. c) both strands can have any of four possible bases attached. d) all the bases of one strand are paired up with their complementary bases on the other strand.

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?