懂题目，却不懂答案！

Xiaotongdi

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
   这题我是读懂了！但是答案是A，我没懂！
   SA会引起M，但是相似的SS就可以通过替代细菌来清走SA，然而SS还是不能作为一个治疗方法，因为……A.某些正常下无法扎根的危险细菌，可以在SS前扎根下来？
   

Xiaotongdi

哇，顿时觉得这题巨简单！

Xiaotongdi

哦哦……对滴对滴！我误解了意思！现在懂了，就错在这个关键上！

kikisunflower

楼主是不是误解了in the presence of的意思

cdwayne

certain dangerous bacteria that normally cannot take hold can do so in the presence of S. simulans

差不多對了，應該是說，原本無法繁殖的有害細菌會因為SS的關係反而開始繁殖

take hold, 扎根, 我説的繁殖。

kikisunflower

就是说打了SS以后，牛更容易感染其他细菌了