Upon the completion of this paper, I would like to give my gratitude to all those who have granted me help on the paper and on my four-year study in this My deepest gratitude goes first and foremost to my supervisor, Professor for her constant encouragement and intellectual guidance and invaluable She has walked me through all the stages of the writing of this Without her consistent and illuminating instruction and valuable assistance, this thesis could not have reached its present Second, I would like to express my heartfelt gratitude to all the friends in this school and companies who have helped me finish the Without their kind help, I could not have completed the I am also greatly indebted to my dear friends and classmates, whose persistent encouragement were often the sustaining force that kept me
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