Acknowledgement

I want to acknowledge the invaluable help of my tutor Professor Liu Yu, who has given my constant consultant in my paper writing, and has guided me and commented through out the whole process of the papers. I am extremely grateful for the valuable advices that she has offered in the creation of the paper, and the great supports she has given in my paper writing. She was very strict with my paper, and checked my paper with great attention several time. She gave many an advice on correcting the paper, and rechecked my paper each time after my correction. I owe my tutor Professor Liu Yu for all the work she has done.