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: Social & Preventive Medicine
A diagnostic test for a paicular disease has a sensitivity of 0.90 and a specificity of 0.90. A single test is applied to each subject in the population in which the diseased population is 10%. What is the probability that a person positive to this test, has the disease - 
Select and explain. 
A. 90%, B. 81%, C. 50%, D. 91%
B: 81%
Answer: B: 81% 
Explanation: Ans. is 'c' i.e., 50%