请教gwd-21-21

Which of the following most logically completes the passage?

Mastitis is an infection of the udder in cows that, although not dangerous, causes them to give poor-quality milk.  Most cases of mastitis are caused by the bacterium Staphylococcus aureus, against which antibiotics are ineffective.  However, a related bacterium, Staphylococcus simulans, quickly clears up even severe infections of S. aureus simply by displacing the bacteria.  S. simulans is nevertheless unlikely to become the preferred treatment for cases of mastitis, since ______.

1. certain dangerous bacteria that normally cannot take hold can do so in the presence of S. simulans
2. the current resistance of S. aureus to antibiotics derives in part from a past pattern of overuse of those antibiotics
3. the milk from cows infected with S. aureus is generally not of such poor quality as to be entirely unusable
4. careful farming practice can reduce the incidence of mastitis to a minimum
5. the only symptom of mild cases of mastitis is the deterioration in the quality of the milk produced

   Answer: a

答案A中的take hold究竟是什么意思？A的逻辑推理是什么，没看懂，请牛人帮忙看看。

take hold = take control

It is saying that Ss can get rid of Sa, but then if you use it, another even more dangerous bacteria will get rid of Ss and take its place to infect  cows.

hence, A is right

Take hold
Longman:   to start to have a definite effect.

If SS is used, the other dangerous bacteria will start to have effect.