题目

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。 Each year there is an increasing number of cars on roads andstreets as millions of new cars and trucks are produced. One out of every sixAmericans work at putting together the parts of cars, driving trucks, buildingroads or filling cars and trucks with gas. Because most Americans find they areusing their cars in almost every aspect of their life. The polluted air becomes poisonous anddangerous to health. That's what several of the large carfactories have been trying to7．计算：$\sqrt{27}$+$\sqrt{48}$=7$\sqrt{3}$．