<P> Within just a decade suburbs dramatically increased in size . Harrow Weald went from just 1,500 to over 10,000 while Pinner jumped from 3,000 to over 20,000 . During the 1930s, over 4 million new suburban houses were built, the' suburban revolution' had made England the most heavily suburbanized country in the world, by a considerable margin . </P> <P> Boston and New York spawned the first major suburbs . The streetcar lines in Boston and the rail lines in Manhattan made daily commutes possible . No metropolitan area in the world was as well served by railroad commuter lines at the turn of the twentieth century as New York, and it was the rail lines to Westchester from the Grand Central Terminal commuter hub that enabled its development . Westchester's true importance in the history of American suburbanization derives from the upper - middle class development of villages including Scarsdale, New Rochelle and Rye serving thousands of businessmen and executives from Manhattan . </P> <P> The suburban population in North America exploded during the post-World War II economic expansion . Returning veterans wishing to start a settled life moved in masses to the suburbs . Levittown developed as a major prototype of mass - produced housing . </P> <P> Very little housing had been built during the Great Depression and World War, except for emergency quarters near war industries . Overcrowded and inadequate apartments was the common condition . Some suburbs had developed around large cities where there was rail transportation to the jobs downtown . However, the real growth in suburbia depended on the availability of automobiles, highways, and inexpensive housing . The population had grown, and the stock of family savings had accumulated the money for down payments, automobiles and appliances . The product was a great housing boom . Whereas, an average of 316,000 new housing non-farm units should have been constructed 1930s through 1945, there were 1,450,000 annually from 1946 through 1955 . The G.I. Bill guaranteed low cost loans for veterans, with very low down payments, and low interest rates . With 16 million eligible veterans, the opportunity to buy a house was suddenly at hand . In 1947 alone, 540,000 veterans bought one; their average price was $7300 . The construction industry kept prices low by standardization--for example standardizing sizes for kitchen cabinets, refrigerators and stoves, allowed for mass production of kitchen furnishings . Developers purchased empty land just outside the city, installed tract houses based on a handful of designs, and provided streets and utilities, or local public officials race to build schools . The most famous development was Levittown, in Long Island just east of New York City . It offered a new house for $1000 down, and $70 a month; it featured three bedrooms, fireplace, gas range and gas furnace, and a landscaped lot of 75 by 100 feet, all for a total price of $10,000 . Veterans could get one with a much lower down payment . </P>

Which of the following was a main reason for the growth of suburbia