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: Forensic Medicine
Which of the following is used to detect old blood stain in a murder scene? 
Select and explain. 
A. Mixed agglutinations, B. Precipiting test, C. Latex test, D. Benzidine test
D: Benzidine test
Answer: D: Benzidine test 
Explanation: Precipitin Test and Latex test are used to differentiate whether human blood or animal blood. Benzidine test is used to detect old blood stains. A combination of two immunologic tests, mixed agglutination and immunofluorescence, that permits the detection of two AB0 system antigens in the same cell within one process. Such mode better validates the conclusion on the group appuenance of cells in analyses of mixed stains originating from different subjects with different blood groups.