A substance has a half-life of two days. If 10 days have passed, how many half-lives have the substance gone through? show your work!!

a- all cells come from pre-existing cells

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms

the sun creates it's own light, also gives light and life to bunch of stuff like plants,animals,humans etc.photosyntesis is the process of making food which needs sun,water,nutrients etc.