The correct answer is (c) Q = 640 C.A. [h(T-t)]^½
To explain: The rate of flow of air arising due to the difference in temperature of outside and inside is given by the expression Q = 640 C.A. [h(T-t)]^½. Here, C stands for coefficient of effectiveness, Q is the rate of air flow in m^3/h, A is the free area of the inlet opening, h is the vertical height difference between inlet and outlet, T is the average temperature of inside air and t is the average temperature of outside air in degree Celcius.