Match the List-I with List-II.
Choose the correct answer from the options given below:
1. Triatomic rigid gas (A): For a rigid triatomic gas, the ratio \( \frac{C_P}{C_V} \) is \( \frac{5}{3} \) because there are no additional degrees of freedom for rotation or vibration. Thus, A matches with I.
2. Diatomic non-rigid gas (B): For a non-rigid diatomic gas, the ratio \( \frac{C_P}{C_V} \) is \( \frac{7}{5} \), so B matches with II.
3. Monoatomic gas (C): For a monoatomic gas, the ratio \( \frac{C_P}{C_V} \) is \( \frac{4}{3} \), corresponding to C matching with III.
4. Diatomic rigid gas (D): For a rigid diatomic gas, the ratio \( \frac{C_P}{C_V} \) is \( \frac{9}{7} \), matching with D matching with IV.
Final Answer $\text{A-III, B-IV, C-I, D-II}$.