This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Phase-contrast microscopy is based on the principle of: 
Select and explain. 
A. Different refractive indices of object, B. Different reflective indices of object, C. Light scattering, D. Light attenuation
A: Different refractive indices of object
Answer: A: Different refractive indices of object 
Explanation: Phase-contrast microscopy takes advantage of minute refractive index differences within cellular components and between unstained cells and their surrounding aqueous medium to produce contrast in these and similar transparent specimens.