Mastitis is an infection of 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 treantment for the cases of mastitis, since_____________.
A. certain dangerous bacteria that normally cannot take hold can do so in the presence of S.simulans
B. the current resistance of S.aureus to antibiotics derives in part from a past pattern of overuse of those antibiotics
C. the milk form cows infected with S.aureus is generally not of such poor quality as to be entirely unusable
D. careful farming practice can reduce the incidence of mastitis to a minimum
E. the only symptom of mild cases of mastitis is the deterioration in the quality of the milk produced
take hold是什么意思，A为什么对呢？ | 
