This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thalassemia, Subject name: Medicine
[{'content': 'HbH is characterized by? \nSelect and explain. \nA. Deletion of three alpha chain genes, B. Deletion of three alpha chains and one beta chain genes, C. Deletion of two alpha and two beta chain genes, D. Deletion of four alpha chain genes', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Alpha -Thalassemia Silent carrier state 1 gene deletion -a a a a thalassemia trait 2 gene deletion -a-a HbH disease 3 gene deletion - - -a Hydrops fetalis 4 gene deletion - - - -', 'role': 'assistant'}]