This book is based on my PhD work. In conducting the research I acknowledge gratefully the cooperation of a number of individuals. First I express my debt of gratitude to Professor Emajuddin Ahamed, former Vice-Chancellor of Dhaka University, who offered me his kind guidance. My indebtedness is due to my colleagues at Jahangirnagar University for their sincere cooperation and valuable suggestions. I am also appreciative of the help of Professor Abu Taher Mojumder. I am thankful to Jahangirnagar University for granting me permission to pursue my research.