Hakeem is testing the effects of sunlight on samples of soil. which of these actions would introduce confounding variables into hakeem’s experiment?
he tests a theory that’s already been established.
he tests multiple hypotheses at a time.
he uses identical specimens when completing multiple trials.
he adjusts the independent variable and records the effects.
he uses different types of soil in each of his samples.

A confounding variable refers to an external effect, which modifies the influence of an independent and dependent variable. This extraneous effect is used to affect the outcome of an experimental design. The confounding variables can ruin the experiment and can generate unnecessary outcomes.  

In the given case, confounding variables can be introduced by testing multiple hypotheses at a time and by using distinct kinds of soil in each of the samples.  

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.