When a chalk is broken into dust-like particles, the particles take a long time to settle down to the ground when compared to the original chalk although the density of the chalk and the viscosity of air remain the same. The reasons for this are: (i) (ii) (i) (ii) The Reynolds number is very low due to reduced length scale of the dust. The dust particle is much lighter than the chalk. The viscous force is a surface force proportional to the length square while the gravitational force is a body force proportional to the length cube. At very small length scale, length square is much larger than length cube. (iii) (A) (B) The largest animal is found in sea as it requires no energy to support its weight by using hydrostatic principle. However bird has to support its own weight by using Bernoulli principle to create pressure difference while in flight. Bird cannot be very large and there are also fewer species of large bird than small bird. The reasons are: There is less food in the air than in the sea. The bone structure of the bird has to be light and it is structurally more difficult to maintain the strength to weight ratio as the bird becomes larger. (i) and (ii) (i) and (iii) (ii) and (iii) All of the above The lift and drag on a bird is due to viscous and pressure forces. They are surface forces proportional to the length square while the weight of a bird due to gravity is proportional to the length cube. At large length scale, length square is much smaller than length cube and it becomes difficult to generate lift to support a bird in flight. (A) (B) (D) (i) and (ii) (i) and (iii) (ii) and (iii) All of the above