Answer:
18 is A
19 is B
20 is B
Step-by-step explanation:
18 is A because the endocrine system is basically the only system that sends chemicals, mainly hormones throughout the body, and the nercous system is the only system that acts like wiring because of the nerves
19 is B because the nervous system needs the brain to function, the nerves send signals to the brain, and the brain sends signals back through the brain stem
20 is B because it can't be A, since that is multiple tissues and organs, it can't be C because cells are what make up tissues, and it can't be D since organ systems require organs not just tissues
hope this helped :)
