单选题 　Passage 3 　　During the early American colonial years, corn was more plentiful than wheat, so corn bread was more common than wheat bread. Friendly Indians showed colonists how to grow corn and how to prepare it for food and pioneer women then improved the Indian cooking techniques. When people traveled, they went to foot or horseback, sleeping and eating in the forests. They carried corn bread for sustenance; the corn bread came to be called journey cake. Later when roads and taverns were built and stagecoaches carried passengers, journey cake became johnnycake, a name many easterners still use for corn bread. The kinds of bread made with cornmeal were-and still are-almost without limit. Every region has its specialities. 　　From the start, southerners showed a preference for white cornmeal, northerners for yellow. And pioneers on the frontier, when then ran out of yeast, made salt-rising bread. They stirred together water, a little water-ground cornmeal, potatoes, and salt. they set the mixture, uncovered, in a warm place until it absorbed bacteria form the air and began to ferment. Then they removed the potatoes and used the liquid as leavening for their bread, made with white flour. In colonial times, why was corn bread more common than wheat bread?