单项选择题
A. hidden B. new C. mysterious D. bare
A．Spiders were the first life that could live on the island. B．The island is far away from any piece of land. C．Insects could not live on the island without plants. D．Plants were brought to the island by human beings.
A．Settlements Spread Westward B．The Coast-to-Coast Railroad: A Vital Link C．American Railroad History D．The Importance of Railroads in the American Economy