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
Following is an example of unusual base: 
Select and explain. 
A. Dihydrouracil, B. Adenine, C. Guanine, D. Uracil
A: Dihydrouracil
Answer: A: Dihydrouracil 
Explanation: Dihydrouracil is an unusual base. The usual bases: Adenine guanine thymine uracil cytosine