Final answer:
The correct answer is C. molecular clock. A molecular clock is a tool used to estimate the time at which species diverged from a common ancestor based on DNA or protein differences.
Step-by-step explanation:
The correct answer is C. molecular clock.
A molecular clock is a tool used to estimate the time at which species diverged from a common ancestor based on DNA or protein differences. It measures the rate of genetic change over time and uses this information to calculate how long it has been since the species diverged.
In the case of HIV-1 M, scientists have used the molecular clock to estimate that it first spread to humans in the 1930s. By comparing the genetic sequences of different HIV-1 M strains and how they have changed over time, researchers can determine when the virus first jumped from its animal host to humans.