(i) To reach the centre of the earth one will have to dig a hole 6000 6000km deep on the ocean floor. This is quite impossible and therefore we cannot go to the centre of the earth.(ii) Rocks break down into small fragments known as sediments. These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks known as sedimentary rocks.(iii) Limestone is changed into marble as a result of metamorphism under great heat and pressure.