This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pharmacodynamics, Subject name: Pharmacology
Calculate the therapeutic index of the drug from given DRC. 
Select and explain. 
A. 8, B. 21, C. 128, D. 1024
B: 21
Answer: B: 21 
Explanation: Therapeutic index is calculated as: TI = LD50/ED50 TI = 128 mg/6 mg = 21.3