Most people usually traveled by ship and train which are driven by steam engine. It played an important part in many kinds of vehicles several scores of years ago. Who invented steam engine and what units could be used to measure the power of engine?　　The word “horse-power” was first used two hundred years ago. James Watt from a worker’s family made the world first widely used steam engine. At first, he couldn’t tell people how powerful it was, because there were no units at that time. Watt decided to find out how much work one strong horse could do in one minute. He named that unit one horse-power. In this way he could measure the work of his steam engine.　　He discovered that a horse could lift a 3300-pound weight 10 feet into the air in one minute. His engine could lift a 3300-pound weight 100 feet in one minute. Because his engine did ten times as much work as the horse, Watt called it a ten horse-power engine.Choose the right answer:The main idea of the passage is ___.