Task: Question Answering
Input (English): Question: The immersion oil used for microscopic observation: 
 Options:  <1> Protects the sample from the oxidizing effect of air. <2> Lubricate the objective lens. <3> Reduces refraction and increases resolution. <4> It acts as a lens that increases magnification. <5> Avoid dehydration of the preparation.
Output (English): 3