题目

Now many young people are traveling around the world on their own, not because they have no one to travel with, but because they prefer to go alone.Kristina Wegscheider from California first traveled alone when she was at college. She believes that it is something everyone should do at least once in their life. It opens up your mind to new things. Wegscheider has visited 46 countries.In foreign countries, with no one to help you read a map, look after you if you get ill, or lend you money if your wallet is stolen, it is challenging. This is what makes young people travel alone. It is seen as c如图4-3-3是古代农村中的一种舂米工具.O为固定转轴，石块固定在A端，脚踏左端B可以使石块升高到P处，放开脚，石块会落下打击稻谷.若脚用力F，方向始终竖直向下，假定石块升起到P处过程中每一时刻都处于平衡状态，则 A.F的大小始终不变                                                    B.F先变大后变小C.F的力矩先变大后变小                                              D.F的力矩始终不变