(i) It is caused by the deficiency of iodine in the diet, needed for synthesis of thyroid hormones. It causes enlargement of the thyroid gland. 

(ii) The water potential of pure water is highest, considered as zero at normal temperature and pressure. All the water molecules are free and have maximum kinetic energy. Addition of solute decrease the kinetic energy of the water molecules (due to collision of solute and solvent molecules), thus lowering the water potential of water in the solution. 

(iii) When we turn round and round, the endolymph in the semicircular ducts, does not move as fast as our body and the sensory cells of cristae continue to move, after the body stops moving. Because of this difference in the rate of movement, the sensory hair of cristae are swept through the endolymph and become bent over, to stimulate sensory cells, which set up action potential in the fibres of auditory nerve, which transmit it to brain.

The brain sends, instructions to muscles to act, hence we lose our balance.

Or 

If you turn round and round in circles, the endolymph in semicircular canal is forced to one end. When you stop moving, the endolymph rushes back, the other way. This give you the whirling sensation in the opposite direction and you feel giddy and find difficult to overcome the tendency of falling down. Thus we lose our balance.

(iv) The spores of Bacillus thuringiensis are used as bio-insecticides because they contain proteinaceous toxins- thurioside as crystals to eradicate different group of insects such as moths, flies, mosquitoes, beetles etc. ingesting the spores. These toxins are converted in to active form and kill the insects by inhibiting the ion transport chain in the midgut.

(v) Owls have better night vision than day vision because they mainly contain rod cells in their retina. These cells possess a visual purple pigment called Rhodopsian which works in dim or diffused light. In bright light, it is broken and does not work.