I would like to express my sincere gratitude to my major professor, Dr. Tang, for her patiently guidance and professional support. She gave me inspirations and instructions to help me complete my research work. Besides my major professor, I also would like to thank my committee members, Dr. Zheng and Dr. Olson, for leading me finish the course work and providing me precious advices to improve my research quality.I thank Jiyeon Jeon and Xi Wang, who gave me guidance on collecting and analyzing data. Because of their support, my thesis got more reliable result.Last but not the least, my sincere thanks also go to my family, for their understanding and offering me the opportunity to study in the master program, without them, this thesis would not have been possible.