Researchers Obtained the Data Depicted in This Figure by Sampling

Multiple Choice

Researchers obtained the data depicted in this figure by sampling human blood containing HIV and then isolating and sequencing genes within the human immunodeficiency virus's DNA .What does the y-axis represent?

A) an index of natural selection on the HIV genes studied
B) an index of mutations within gene regions of HIV that are unrelated to the success of the virus (such as how virulent it is or how well it reproduces within its human hosts)
C) a measurement of changes between HIV gene sequences,specifically on genes important for the replication,reproduction,and spreading of the virus
D) an index of time since the first human HIV infection occurred