Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.

         The economic expansion prompted by the Second World War triggered a spectacular population boom in the West. Of course, the region was no stranger to population booms. Throughout much of its history, western settlement had been characterized by spurts, rather than by a pattern of gradual and steady population growth, beginning with the gold and silver rushes of the 1850's and 1860's. The decade after the First World War - the 1920's - witnessed another major surge of people pouring into the West, particularly into urban areas. But the economic depression of the 1930's brought this expansion to a halt; some of the more sparsely settled parts of the region actually lost population as migrants sought work in more heavily industrialized areas. By 1941 when the United States entered the Second World War and began to mobilize, new job opportunities were created in the western part of the nation.

          If the expansion of industries, such as shipbuilding and aircraft manufacturing, was most striking on the pacific coast, it also affected interior cities like Denver, Phoenix, and Salt Lake City. Equally dramatic were the effects of the establishment of aluminum plants in Oregon and Washington and the burgeoning steel industry in Utah and California. The flow of people into these areas provided an enormous impetus to the expansion of the service industries - banks, health care services and schools. Although strained to the limit by the influx of newcomers, western communities welcomed the vast reservoir of new job opportunities. At the same time, the unprecedented expansion of government installations in the West, such as military bases, created thousands of new civilian openings. As land had served as a magnet for western migrants in the late nineteenth century, so wartime mobilization set in motion another major expansion of population. Indeed, it could be said that the entire western United States became a giant boomtown during the Second World War. This was especially true of California. Of the more than eight million people who moved into the West in the decade after 1940, almost one-half went to the Pacific coast. In fact, between 1940 and 1950, California's population surged by more than three million people.

It can be inferred from the passage that the principal cause of California’s population surge between 1940 and 1950 was ______.