Answer: A and D
Explanation: pretty simple
The answer for this question would be Integument, digestive, respiratory and urogenital.
The largest system that contact with the external environment would be integument and digestive. That makes two system has normal flora of microbes and not sterile like the other systems. Normal flora of microbes will prevent the pathogen microbe grows so it gives some benefit.
Answer:
By exercising, it increases blood flow towards the muscles and digestive tract. This can help move food through the stomach, or digestive tract.
