The author of this passage probably intended to explain ,______.
A．exactly how radar worksB．why the British used radio waves in their deviceC．how radar(word and device)came to beD．how radar helped the British win the Battle of Britain

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 lightsB．the shortage of busesC．the lack of motorwaysD．the traffic congestion

呕血后患者出现休克表现时则失皿量应在()
A.失血量约为血容量的15%左斋B.血量约为血容量的20%左右C.失血量约为血容量的25%左右D.失血量约为血容量的E.失血量约为血容量的35%左右

The other day, Mum and I went to St. James"s Hospital, and they did lots and lots of tests on me, ______are horrible and frightening.
A．most of themB．most of whichC．most of thatD．most of what

Many countries are increasing their use of natural gas, wind and other forms of______.
A．energyB．sourceC．powerD．material

The word "arid" is closest in meaning to ______.
A. sunnyB. flatC. dryD. sandy