Big cities today are confronted with very serious problems. Transport is a【C1】______difficulty: some planners believe in【C2】______transport system; others believe in making it possible for citizens to move【C3】______in private cars. There are several ways of【C4】______a public transport system. Sometimes it is built under the ground; sometimes it is on the【C5】______, and sometimes it is even above ground level,【C6】______in parts of New York and Tokyo for example. In most cases, of course, it is a【C7】______of some or all of these elements. The【C8】______of the city centers has also been a problem in the last 30 years. The hearts of many cities have become poorer and poorer,【C9】______more and more crime and violence, 【C10】______the people who can afford to have moved out to the【C11】______. This is the opposite of the situation 100 years ago,【C12】______wealthy people lived in the center, and the poor lived【C13】______the outskirts of the city. Ecological problems also【C14】______, as factories and cars pollute the air, trees are cut down to make【C15】______for urban clearways, and crowded conditions cause garbage to【C16】______in the streets. In order to solve these problems, some planners believe we should limit the size of our cities, perhaps by restricting population, or perhaps by offering【C17】______jobs in the provinces. Others believe that cities cannot be limited in size and that the problems must be solved in a【C18】______of ways—by creating green areas, building【C19】______houses for everybody, stopping factory pollution,【C20】______motor transport, and so on."