Varieties of staphylococcus aureus that are resistant to the drug methicillin 1) developed in response to the use of methicillin 2) already existed in the population before methicillin was developed 3) always have an advantage in every environment 4) were strongly selected for as methicillin became widely used to treat bacterial infections 5) the second and fourth answers are correct.

Option 2

Explanation:

MRSA stands for Methicillin-Resistant Staphylococcus aureus  is a bacterium responsible for diseases in various body parts.  

It's harder to treat as compared to most of the staphylococcus aureus strains  - or staph - in light of the fact that it's impervious to some usually utilized anti-infection agents, i.e., antibiotics.

Their existence in population was even before the development of Methicillin.