In June 1956 a TWA Constellation collided with a United Air Lines DC-7 over the Grand Canyon in Arizona,killing all 128 people on both aircraft.At the time it was the worst ever airline disaster Struggling with outdated technology and a post-war boom in air travel overworked air-traffic controllers failed to spot that the planes were on a collision course.（1）With that system again struggling to keep pace with demand,Donald Trump thinks it is time to privatise Americas air-traffic control service.This week the president out-lined a plan to turn air-traffic control into a separate non-profit entity financed by user fees,instead of the present patchwork of taxes and grants.Shorn of its air-traffic responsibility,the FAA would become a satety body America's air-traffic system is vast,consising of 14,000 controllers working in 476 airport-control towers that handle take-offs and landings,as well as in 21"en route"centres looking after flights along the nation's airways.（2）This leads to delays and frustrated flyers.With passenger numbers set to grow from 800m a year to almost Ibn by 2026,the problem will only get worse（3）Many countries,including Australia,Britain and Canada,have privatised air-traffic services or turned them into state-owned firms.Nav Canada,a non-profit firm that has long managed Canadian airspace,has costs per flight hour of 340 compared with the FAA's$450.Replacing old radar-based methods with accurate satellite navigation and better digital communications is a particular priority.Aircraft using satellite navigation can be safely spaced more closely together,which permits many more planes to be in the air at the same time.Digital systems also provide data links to control centres and to other planes by regularly broadcasting an aircraft's identification sign,its position and course.（4）The president's proposal might even speed a move towards"virtual"control towers in low-rise buildings,which can replace towers physically localed at airports.The European Union reckons such innovations will allow three times as many flights to be handled in the region and save airlines some 9bn（$10bn）a year.It also,optimistically perhaps,predicts that on average aircraft will land with in one minute of their scheduled arrival time.That would count as a miraculous improvement for any one,let alone America's weary airport warriors（5）A similar plan got stuck last year,despite being backed by most airlines and the air-traffic controllers'union.At least the president can dodge the queues Air Force One flights get special clearance
2选？
本段开头指出,美国的航空交通管理系统规模庞大。下文指出,这导致了航班的延误,使得乘客深感沮丧不满。由此可知,空白处内容为该系统的弊端,与上文内容成转折关系。[B内容为,这个系统有着良好的安全记录,但老旧的技术限制了系统可处理的航班数量,本句开头的it指代美国的航空交通管理系统,且该句与上文成转折关系,故为答案
