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: Ophthalmology
Rigid gas permeable lens are made of- 
Select and explain. 
A. Plymethymethacrylate, B. Hydroxymethylmethacrylate, C. Co polymer of PMMA, Silicon containing monomer & cellulose acetyl butyrate, D. Cellulose acetae Butyrate
C: Co polymer of PMMA, Silicon containing monomer & cellulose acetyl butyrate
Answer: C: Co polymer of PMMA, Silicon containing monomer & cellulose acetyl butyrate 
Explanation: 1. Hard lenses: Made of pokymethylmethacrylate (PMMA) 2. Rigid gas permeable (RGP) lenses: Copolymer of PMMA, silicon containing vinyl monomer & cellulose acetate butyrate (CAB) are used to manufacture RGP lenses. 3. Soft lenses: These are made up of hydroxymethymethacrylate (HEMA)