Alvin is studying a model of the chemical structure of a common nucleic acid. He observes that the nucleic acid consists of four nitrogenous

bases: adenine (A), cytosine (C), guanine (G), and uracil (U).

Alvin's observation most strongly supports which of these conclusions?

A. The nucleic acid is a form of RNA, and is not DNA.

B. The nucleic acid is either mRNA or rRNA, but not tRNA.

C. The nucleic acid is either RNA or DNA.

D. The nucleic acid is DNA, and is not RNA.

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?