63. a student wanted to determine if slugs preferred green leaf lettuce
leaves over purple cabbage leaves for food. pieces of both leaves
were cut. one piece of each type of leaf and one slug were placed in
each often containers. after three days, the surface area of each leaf
section was measured and the results were recorded in a data table.
state one reason that the results of this experiment might be considered
invalid.

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?