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
Objective of ETDRS study is 
Select and explain. 
A. Effect of aspirin on the course of diabetic retinopathy, B. To observe patients with severe DR in type 1 and type 2 diabetes over 2 years to determine visual outcomes, C. To evaluate whether photocagulation prevents severe visual loss in eyes with diabetic retinopathy, D. ALL of the above
A: Effect of aspirin on the course of diabetic retinopathy
Answer: A: Effect of aspirin on the course of diabetic retinopathy 
Explanation: Option A - ETDRS study
Option C - DRS study
Option B - DRVS study