(a) Firki rotates when the force is applied to the blades of the first. The force is applied on the blades of the first by the moving air called wind but in the closed area there is lack of air movement (wind) so firki does not rotate in a closed area.

(b) The arrow of weather cock points towards a particular direction because it tells us about the current direction of the wind(air) at that time.

Or

The arrow of weather cock points towards a particular direction because it tells us about the direction of wind movement.

(c) We can say that an empty glass, in fact, is not empty as it is filled with the air. We cannot see the air but we can feel it as air is transparent and has no weight. Air do not have a fixed shape and fixed volume.

(d) It is right to say that breathing through the mouth may harm us because the air contains dust particles and if we breathe through mouth then the harmful dust particles may enter our body(respiratory system) and cause damage to lungs. But If we breathe through the nostrils then the fine hair and mucus which is present in the nose may prevent the dust particles from entering our body.