ANS:C - both 'a' & 'b'

The use of a natural draft cooling tower is practiced when the air has low humidity. Natural draft cooling towers, also known as hyperbolic cooling towers, rely on the stack effect to induce airflow through the tower. As warm water from industrial processes is circulated through the tower, some of it evaporates, cooling the remaining water. The warm, moist air rises naturally due to the difference in temperature and density between the inside and outside of the tower. As it rises, it creates a draft that draws in cooler, drier air from the surroundings at the bottom of the tower. Low humidity in the surrounding air is beneficial for natural draft cooling towers because it allows for more efficient evaporation of water from the tower. When the air is dry, it has a greater capacity to absorb moisture, enhancing the cooling effect within the tower. Temperature alone does not determine the effectiveness of a natural draft cooling tower. However, lower temperatures can contribute to a greater temperature differential between the inside and outside of the tower, which can enhance the stack effect and improve airflow. Therefore, the correct answer is neither 'a' nor 'b'. While low humidity is particularly beneficial for natural draft cooling towers, temperature alone does not determine their effectiveness.