Which statement is true?

Options

A. adenine has 4 nitrogen atoms
B. cytosine has 3 nitrogen atoms
C. guanosine has 3 nitrogen atoms
D. uracil has 5 nitrogen atoms

Explanation

To determine which statement is true regarding the nitrogenous bases, we will analyze each option step by step.

### Step 1: Analyze Option 1 - Adenine has 4 nitrogen atoms.
- Adenine is a purine base, which is a double-ring structure. It contains a total of 4 nitrogen atoms located at the 1, 3, 7, and 9 positions of its structure.
- **Conclusion**: This statement is true.

### Step 2: Analyze Option 2 - Cytosine has 3 nitrogen atoms.
- Cytosine is a pyrimidine base, which is a single-ring structure. It contains only 2 nitrogen atoms located at the 1 and 3 positions.
- **Conclusion**: This statement is false.

### Step 3: Analyze Option 3 - Guanine has 3 nitrogen atoms.
- Guanine is also a purine base, similar to adenine, and it has 4 nitrogen atoms located at the 1, 3, 7, and 9 positions.
- **Conclusion**: This statement is false.

### Step 4: Analyze Option 4 - Uracil has 5 nitrogen atoms.
- Uracil is a pyrimidine base and has only 2 nitrogen atoms located at the 1 and 3 positions. It is the demethylated form of thymine and does not contain 5 nitrogen atoms.
- **Conclusion**: This statement is false.

### Final Conclusion:
The only true statement among the options provided is that **adenine has 4 nitrogen atoms**.

---