This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and refraction, Subject name: Ophthalmology
The material of soft lens ? 
Select and explain. 
A. Polymethylmethacrylate, B. Hydroxymethylmethacrylate, C. Silicon, D. Cellulose acetate Butyrate
B: Hydroxymethylmethacrylate
Answer: B: Hydroxymethylmethacrylate 
Explanation: Types of contact lenses- Depending upon the nature of the material used in their manufacturing, the contact lenses can be divided into three types:- Hard lenses: Made of polymethylmethacrylate (PMMA) Rigid gas permeable (PGR) lenses:- Copolymer of PMMA, silicon-containing vinyl monomer and cellulose acetate butyrate (CAB) are used to manufacture RGP lenses Soft lenses:- These are made up of hydroxymethyl methacrylate (HEMA). Reference : AK Khurana Comprehensive Ophthalmology, 6th edition, pg no. 208 - 209