据常识可知选C。

　　18．A.affect B．use

　　C．save D．share

　　解析：选C　我们现在应该"节约(save)"每一寸土地。

　　19．A.Otherwise B．However

　　C．Therefore D．Though

　　解析：选A　otherwise"否则"，符合语境。

　　20．A.people B．globe

　　C．population D．number

　　解析：选C　用large修饰population，指人口多。

　　Ⅱ.阅读理解

　　Self­driving cars might make your future travel a lot more pleasant, but they won't reduce (缓解) traffic.

　　Car producers have mentioned that traffic reduction is one of the many advantages of having self­driving cars on the road. The idea is that self­driving cars will reduce accidents caused by human mistakes. But experts say self­driving cars' influence on traffic will probably be bad.

　　Lew Fulton, a co­director at UC Davis' Institute of Transportation Studies (ITS), said that self­driving cars won't fix traffic problems unless a pricing system is put in place that discourages self­driving cars. For example, many companies are interested in programming self­driving cars to do something, which could increase traffic on the road, Fulton said.

　　Massachusetts lawmakers have already suggested introducing a tax (税) on driverless cars. It calls for at least $0.025 per mile.

　　Traffic jams (堵塞) could also worsen as companies like Lucid Motors try to produce self­driving cars with reclining seats (可躺座椅). People may choose to live farther outside of cities if they can travel in cars that allow them to sleep and relax. But that increases the number of people travelling in and out of cities during rush hour, Fulton said.

　　Without the cost of a driver, Fulton said he worries self­driving Ubers or Lyfts will become so cheap it will be hard to push people to choose car­sharing services like uberPOOL. "I think it's going to take some kind of pricing system that discourages cars without a single passenger，" he said.

Some cities have already started programmes that discourage people from taking car trips alone. For example, Chicago and New York City have set up lanes (车道) that single­passenger cars must pay some money to use. That money increases during rush hour.