When something is on fire, people throw sand on it to put out the fire. The main use of the sand is to _________.

A．protect the thing on fire

B．add weigh to the thing on fire

C．keep the air away from the thing on fire

D．lower the temperature of the thing on fire

炉料透气性与煤气流运动极不适应，炉料停止下降的失常现象就是（）。