Anything that causes a reaction is considered a stimulus true or false?

True

Explanation:

Stimulus refers to any changes in the internal or external conditions of the organisms. Organisms sense the stimulus with the help of sense organs and respond in a certain way. For example, there is an increase in breathing rate during exercise to meet the increased demand for oxygen by muscles in human.

a- all cells come from pre-existing cells

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.