Why is it necessary that only one variable be changed in an experiment,
and that all other variables remain constant?

According to the loco mass the mass of reactants and products

How is ribosomal rna useful as a molecular clock? a. a large portion of the dna ring is not vital to structure or function, allowing it to accumulate neutral mutations. b. its rate of mutation increases over time as organisms continue to evolve and differentiate from each other. c. a slow mutation rate makes it useful for determining evolutionary relationships between ancient species. d. it is only found in select organisms, making it easier to compare relationships between species that have it.

This is a biological reaction due to a stimulus. i need this quickly !