Passage Three：Questions 21 to 25 are based on the following passage. What might driving on an automated highway be like? The answer depends on what kind of sys tem is ultimately adopted. Two distinct types are on the drawing board. The first is a special purpose lane system, in which certain lanes are reserved for automated vehicles. The second is a mixed traffic system: fully automated vehicles would share the road with partially automated or manual driven cars. A special purpose lane system would require more extensive physical modifications to existing highways, but it promises the greatest gains in freeway (高速公路) capacity. Under either scheme, the driver would specify the desired destination, furnishing this information to a computer in the car at the beginning of the trip or perhaps just before reaching the automated highway. If a mixed traffic system way was in place, automated driving could begin whenever the driver was on suitably equipped roads. If special purpose lanes were available, the car could enter them and join existing traffic in two different ways. One method would use a special onramp (入口引道). As the driver approached the point of entry for the highway, devices installed on the roadside would electronically check the vehicle to determine its destination and to ascertain that it had the proper automation equipment in good working order. Assuming it passed such tests, the driver would then be guided through a gate and toward an automated lane. In this case, the transition from manual to auto mated control would take place on the entrance ramp. An alternative technique could employ conventional lanes, which would be shared by automated and regular vehicles. The driver would steer onto the highway and move in normal fashion to a “transition” lane. The vehicle would then shift under computer control onto a lane reserved for automated traffic. (The limitation of these lanes to automated traffic would, presumably, be well respected, because all trespassers (非法进入者) could be swiftly identified by authorities.) Either approach to joining a lane of automated traffic would harmonize the movement of newly entering vehicles with those already traveling. Automatic control here should allow for smooth merging without the usual uncertainties and potential for accidents. And once a vehicle had settled into automated travel, the driver would be free to release the wheel, open the morning paper or just relax. 第21题：We learn from the first paragraph that two systems of automated highways ________. A) are being planned B) are being modified C) are now in wide use D) are under construction