问题：
[单选,材料题] quantity回答下面的题目：
Traffic in Uur Cities
Tile volume of traffic in many cities in the world today continuesto expand．This causes many problems，including serious air pollution，lengthydelays，and the greater risk__________ (51)accidents．Clearly，something must be done，but it is often difficult to persuade people to__________ (52)theirhabits and leave their cars at home．
One possible__________ (53)is to make it more expensive for peopleto use their cars by__________ (54)charges for parking and__________ (55)tougherfines for anyone who__________ (56)the law．In addition，drivers could be required to pay for using particular routes atdifferent times of the day．This system，__________ (57)as“road pricin9”，is alreadybeing introduced in a__________ (58)of cities，using aspecial electronic card__________ (59)to the windscreen of the car．
Another way of__________ (60)with the problem is to provide cheapparking on the__________ (61)of the city，and strictly control the number ofvehicles allowed into the centre．Drivers and their passengersthen use a special bus__________ (62)for the final stage of their journey,Ofcourse．the most important__________ (63)is to providegood public transport．However，toget people to__________ (64)the comfort of their cars，publictransport must be felt to be reliable，convenient andcomfortable，with fares__________ (65)at an acceptablelevel．
A . total
B . sum
C . number
参考答案：D
参考解析：
根据句意，“道路收费”系统已经被引入一些城市来使用，a number of表示“一些”，符合句意，为正确答案。A项a quantity of表示“大量”，可修饰可数和不可数名词，但侧重大批量计算，含精确测量的意味；B项；total of表示“总计”，a sum of表示“合计，总数”，均不符合句意，故选D。
