单项选择题 Can a small group of drones（无人机）guarantee the safety and reliability of railways and,at the same time, help railway operators save billions of euros each year? That is the very likely future of applying today's "eyes in the sky" technology to making sure that the millions ofkilometres of rail tracks and infrastructure（基础设施）worldwide are safe for trains on a 24/7 basis.Drones are already being used to examine high-tension electrical lines. They could do precisely the same thing to inspect railway lines and other vital aspects of rail infrastructure such as the correct position of railway tracks and switching points. The more regularly they can beinspected, the more railway safety, reliability and on-time performance will be improved. Costs would be cut and operations would be more efficient（高效）across the board.That includes huge savings in maintenance costs and better protection of railway personnel safety. It is calculated that European railways alone spend approximately 20 billion euros a year on maintenance, including sending maintenance staff, often at night, to inspect and repair the railinfrastructure. That can be dangerous work that could be avoided with drones assisting the crews'efforts.By using the latest technologies, drones could also start providing higher-value services for railways, detecting faults in the rail or switches, before they can cause any safety problems. To perform these tasks, drones for rail don't need to be flying overhead. Engineers are now workingon a new concept: the rail drones of the future. They will be moving on the track ahead of the train,and programmed to run autonomously. Very small drones with advanced sensors and AI and travelling ahead of the train could guide it like a co-pilot. With their ability to see ahead, they could signal any problem, so that fast-moving trains would be able to react in time.Which is the most suitable title for the text?

单项选择题 函数的偏导数在点处连续, 是函数在该点处可微分的        条件. A、充分 B、必要 C、充分必要 D、非充分非必要

单项选择题 广州某国际旅行社在深圳设立分社，如果该分社因自身过失造成旅游者权益受到严重损失，旅游者向投诉管理机关进行投诉.那么被投诉者所在地应为()A、广州B、深圳C、广州和深圳D、广州或深圳