This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pharmacology
A 32-yrs-old woman is diagnosed with major depression. What is the chance that her identical twin sister will develop the same disease? (NOT RELATED TO PHARMACOLOGY) 
Select and explain. 
A. 5%, B. 25%, C. 50%, D. 75%
C: 50%
Answer: C: 50% 
Explanation: The concordance rate for mood disorders (Mania) in monozygotic twins averages 70% and for depression it is 50% when various studies are combined. The concordance rate in dizygotic twins is approximately 20%.