3. Briefly discuss the following: (a) What is meant by Frenkel defect and Schottky defect? Briefly explain why Frenkel defect is more likely to occur in crystals. (5 marks) (b) What is the distinction between glass transition temperature and melting temperature? Sketch a graph to show the difference of glass transition temperature between crystalline and amorphous polymers. (5 marks) (c) What are the differences between crystalline materials and amorphous materials? Briefly explain how these two types of material be determined. (5 marks) (d) What are the differences between cement, mortar, concrete and reinforced concrete? (5 marks)