21.Which of the following statements regarding the use of diamond are correct ? 1. Diamond being teh hardest substance, is used in glass cutters and rock drilling equipment. 2. Because of its extraordinary sensitivity to heat rays, diamond is use for making high precision thermometers. 3. Sharp - edged diamonds are used by eye - surgeons as a tool to remove catarct from eyes. 4. Because of its ability to keep out the harmful radiations, diamond is used for making protective windows for space satellites.

22.A glass tumbler cracks when very hot water is poured into it because

23.The blue colour of the water in the sea is due to

24.Pouring of kerosene oil on stagnant water reduces mosquito problem because :

25.The credit of improving the properties of rubber by the process of vulcanization goes to