While the roots of social psychology lie in the intellectual soil of the whole western tradition, its present flowering is recognized to be characteristically an American phenomenon. One reason for the striking upsurge of social psychology in the United States lies in the pragmatic tradition of this country. National emergencies and conditions of social disruption provide special incentive to invent new techniques, and to strike out boldly for solutions to practical social problems. Social psychology began to flourish soon after the First World War. This event, followed by the great depression of the 1930s, by the rise of Hitler, the genocide of Jews, race riots, the Second World War and the atomic threat, stimulated all branches of social science. A special challenge fell to social psychology. The question was asked: How is it possible to preserve the values of freedom and individual rights under condition of mounting social strain and regimentation？ Can science help provide an answer？ This challenging question led to a burst of creative effort that added much to our understanding of the phenomena of leadership, public opinion, rumour, propaganda, prejudice, attitude change, morale, communication, decision- making, race relations, and conflicts of war.
Reviewing the decade that followed World War Il, Cartwright speaks of the "excitement and optimism" of American social psychologists, and notes "the tremendous increase in the total number of people calling themselves social psychologists." Most of these, we may add, show little awareness of the history of their field.
Practical and humanitarian motives have always played an important part in the development of social psychology, not only in America but in other lands as well. Yet there have been discordant and dissenting voices. In the opinion of Herbert Spencer in England, of Ludwig Gumplowicz in Austria, and of William Graham .Sumner in the United States, it is both futile and dangerous for man to attempt to steer or to speed social change. Social evolution, they argue, requires time and obeys laws beyond the control of man. The only practical service of social science is to warn man not to interfere with the course of nature (or society). But these authors are in a minority. Most social psychologists share with Comte an optimistic view of man's chances to better his way of life. Has he not already improved his health via biological sciences7 Why should he not better his social relationships via social sciences？ For the past century this optimistic outlook has persisted in the face of slender accomplishment to date. Human relations seem stubbornly set. Wars have not been abolished, labour troubles have not abated, and racial tensions are still with us. Give us time and give us money for research, the optimists say.
Social psychology developed in the USA

A. because its roots are intellectually western in origin.
B. as a direct response to the great depression.
C. to meet the threat of Adolf Hitler and his policy of mass genocide.
D. because of its pragmatic traditions for dealing with social problems.