The two proteins directly involved in muscle contraction are broadly called . During an exercise where muscles do not shorten, contractions have occurred. a. has an affinity for myosin binding sites in the absence of calcium. b. is a missing protein in Duschenne's muscular dystrophy. c. The cell membrane of a muscle fiber is the .

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?