I think the answer is A. Hope this helps
Answer:
The skin
Digestive tract
Vagina
Mucous membrane
Explanation:
The skin prevents microbes from entering the body
Digestive tract e.g. stomach acid is a natural defence mechanism
The vagina is acidic and this prevents harmful bacteria from growing with it's own flora.
Mucous membranes e.g. lining of the mouth, nose, eyelids are all defense mechanisms.
Answer:With this system, blood always flows in only one direction inside the heart. There are also valves at the bottom of the large arteries that carry blood away from the heart: the aorta and the pulmonary artery. These valves keep the blood from flowing backward into the heart once it has been pumped out.
Explanation:
The period during which adolescents reach sexual maturity and become capable of reproduction