This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nucleotides, Subject name: Biochemistry
[{'content': 'Following is an example of unusual base: \nSelect and explain. \nA. Dihydrouracil, B. Adenine, C. Guanine, D. Uracil', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Dihydrouracil is an unusual base. The usual bases: Adenine guanine thymine uracil cytosine', 'role': 'assistant'}]