单选题：根据以下资料，回答题。　　Years ago our cities were full of cars, buses a

根据以下资料，回答题。 　　Years ago our cities were full of cars, buses and trucks．Now the streets are completely congested (拥挤的) and it is very difficult to drive a car along them．Drivers must stop at hundreds of traffic lights．What are our cities going to be like in ten or twenty years? Will enormous ( 巨大的 ) motorways be built across them? With-big motorways cutting across them, full of noisy, dirty cars and lorries, our cities are going to be awful places．How can we solve the problem?
　　There are some good ideas to reduce the use of private cars．In 1989, for example, the authorities in Rome began an interesting experiment: passengers on the city buses did not have to pay for their tickets.
　　In Stockholm there was another experiment: people paid very little for a season ticket to travel on any bus, trolley bus, train or tram in all the city.
　　In many cities now some streets are closed to vehicles, and pedestrians are safe there.
　　In London there is another experiment: part of the street is for buses only, so the buses can travel fast．There are no cars or taxies in front of them.
What the writer worries about in a big city is __. A． the number of traffic lights
B． the shortage of buses
C． the lack of motorways
D． the traffic congestion