請依下文回答第 31 題至第 35 題 An unmanned aerial vehicle, commonly known as a drone, is an aircraft without a human pilot aboard. About 700,000 drones were sold in 2015, and the number skyrocketed to almost 3 million in 2017 in the U.S. Ordinary people and small businesses use these flying devices to do all sorts of things, from aerial photography to crop-dusting to just having fun. It also enables shipping to remote places and assists search and rescue missions for humans and livestock alike. But the technology has a dark side. Terrorists and criminals could use drones to conduct illegal surveillance, carry out chemical, biological, and radiological attacks as well as more traditional attacks on large open-air venues such as concerts, sporting events and government facilities. It was reported that criminals used small drones to block FBI agents from observing a hostage situation in 2017. Drones could be used to smuggle as well. Last year, the U.S. Border Patrol agents arrested a 25-year- old man for using a drone to ferry drugs across the southwest border. Since October 2017, officials have reported over three-dozen sightings of drones operating near the U.S. border. The U.S. Customs and Border Protection is concerned that drones could show smugglers in Mexico vulnerable points on the border where people and/or drugs can cross over and sneak into the country undetected. A wide variety of counter-drone measures have been developed in response to this security threat. For example, DJI, the world’s leading maker of unmanned aerial vehicles, enforced geofencing, a virtual geographical boundary across most of Iraq and Syria, and stopped terrorists from using its drones in both countries in 2017. Both France and the Netherlands have been quietly training eagles to take down drones deemed as hostile. The Police of Wuhan, China have developed a signal-jamming gun of their own. This gun disrupts the drone’s radio signals and forces it to land. Instead of relying on radio frequencies, SkyWall offers net-based options for polices and governments. It fires projectiles that could be turned into a net to “nonviolently” down the drone.