Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.
Answer:
D. The digestive system breaks food down into the molecules that can be used in respiration.
Explanation:
Yes a hypothesis is an educated guess because you guess on what the conclusion will be in an experiment
Answer: i think it's the first one and coexist is not in the dictionary
