The correct option for the conduction of electricity through gases is **B. I and III only** (Low pressure and High p.d.). ### Detailed Explanation: 1. **Understanding Gas Conduction**: - Gases are generally poor conductors of electricity under normal conditions. However, when certain conditions are met, they can conduct electricity. The two critical factors that influence the conduction of electricity through gases are pressure and potential difference (p.d.). 2. **Low Pressure (I)**: - At low pressures, the density of gas molecules is reduced. This means that there are fewer gas molecules in a given volume, which allows electrons (or ions) to move more freely without colliding with other gas molecules. - When a high voltage is applied, it can ionize the gas, creating free electrons and positive ions. The low pressure facilitates this ionization process because the mean free path (the average distance a particle travels before colliding with another particle) is longer, allowing charged particles to gain energy and move more freely. 3. **High Potential Difference (III)**: - A high potential difference (voltage) is necessary to provide enough energy to ionize the gas. When a high voltage is applied across a gas, it can strip electrons from gas atoms, creating ions. This process is known as gas ionization. - Once ionized, the gas can conduct electricity as the free electrons and ions can move towards the electrodes, allowing current to flow. Thus, a high p.d. is essential for initiating and maintaining the conduction of electricity through gases. ### Why the Other Options are Incorrect: - **Option A: I and IV only (Low pressure and Low p.d.)**: - While low pressure is correct, low potential difference does not facilitate the conduction of electricity through gases. A low p.d. would not provide sufficient energy to ionize the gas, thus preventing conduction. - **Option C: II and IV only (High pressure and Low p.d.)**: - High pressure is incorrect because at high pressures, gas molecules are closer together, leading to more frequent collisions. This increases the likelihood of scattering and reduces the mean free path, making it harder for electrons to move freely. Additionally, low p.d. does not provide the necessary energy for ionization. - **Option D: II and III only (High pressure and High p.d.)**: - While high p.d. is correct, high pressure is not conducive to gas conduction. As explained, high pressure leads to more collisions among gas molecules, which hinders the movement of charged particles. ### Summary of Key Points: - **Low Pressure**: Facilitates free movement of electrons due to fewer collisions. - **High Potential Difference**: Provides the energy needed to ionize the gas and create charge carriers. - **High Pressure**: Inhibits conduction due to increased collisions among gas molecules. - **Low Potential Difference**: Insufficient to ionize the gas and initiate conduction. This understanding is crucial for grasping the principles of gas conduction in physics, especially in applications like gas discharge tubes and neon lights.