Why must a drop of immersion oil be placed between the glass slide and the 100x objective lens when using an oil immersion objective on a light microscope?

• To prevent light rays from refracting and scattering, thereby improving image resolution
• To cool the slide and prevent the high-intensity light source from cooking the bacteria
• To physically adhere the objective lens to the slide so the focus does not drift
• To stain the bacteria on the slide so they are visible under the microscope