Answer:
b
Explanation: it is b because if they stop using the cows or pigs then they are going 2 need 2 have to use something else and it is not dogs or cats because in no where in that it talks about dogs and cats and also it is not that because there is nothing that dogs and cats can do there is nothing in there body that can help the human body and it is E.coli or yeast because our body needs that