Acknowledgements Here and now， I would like to extend my sincere thanks to all those who have helped me make this thesis possible and Firstly， I am deeply grateful to my honorable supervisor， ***， who have checked through my thesis with patience and given me instructive suggestions， and she also played an important role in indicating a bright road in my future Then thanks to the teachers and professors who have taught me over the past three years of Finally， I am very grateful to my lovely friends and classmates who have offered me quiet situation to compose my thesis and discussed with me about my
