Which of the following nitrogenous base is double ringed?

Options

A. Guanine
B. Uracil
C. Thymine
D. Cytosine

Answer

To determine which of the following nitrogenous bases is double-ringed, we need to classify the bases into two categories: purines and pyrimidines.

### Step-by-Step Solution:

1. **Understand the Types of Nitrogenous Bases**:
- Nitrogenous bases are classified into two groups: purines and pyrimidines.
- Purines have a double-ring structure, while pyrimidines have a single-ring structure.

2. **Identify the Purines**:
- The purines include adenine (A) and guanine (G).
- Both of these bases have a double-ring structure.

3. **Identify the Pyrimidines**:
- The pyrimidines include thymine (T), cytosine (C), and uracil (U).
- All of these bases have a single-ring structure.

4. **Analyze the Options**:
- The options given are guanine, uracil, thymine, and cytosine.
- Among these, guanine is the only base that is classified as a purine and has a double-ring structure.

5. **Conclusion**:
- Therefore, the nitrogenous base that is double-ringed is **guanine**.

### Final Answer:
**Guanine** is the nitrogenous base that is double-ringed.

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