Which one of the following ratios is variable but constant for a species?

Options

A. `([A +T])/([G+C])`
B. `([A + G])/([T + C])`
C. `([A + U])/([G+ C])`
D. None of these

Answer

To solve the question, "Which one of the following ratios is variable but constant for a species?", we need to analyze the options provided and apply Chargaff's rule.

### Step-by-Step Solution:

1. **Understand Chargaff's Rule**:
Chargaff's rule states that in a given DNA molecule, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). This can be expressed as:
- A = T
- G = C

2. **Identify the Ratios**:
The question provides several ratios:
- A + T / G + C
- A + T / C
- A + U / G + C
- None of these

3. **Analyze the Ratios**:
- The ratio A + T / G + C represents the total amount of purines (A + G) and pyrimidines (C + T) in the DNA.
- According to Chargaff's rule, while the absolute values of A, T, G, and C can vary between species, the ratio A + T / G + C remains constant for a given species.

4. **Conclusion**:
Therefore, the correct answer is:
- A + T / G + C is variable but constant for a species.

### Final Answer:
The ratio that is variable but constant for a species is **A + T / G + C**.