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
Basic types of biases in epidemiological study-a)  Investigation biasb)  Subject biasc)  Random errord)  Analyzer bias 
Select and explain. 
A. dc, B. cb, C. abd, D. b
C: abd
Answer: C: abd 
Explanation: None