题目

    The United States estimates that about one out of every 10 people on the planet today is at least 60 years old. By 2050, it’s projected to be one out of 5. This means that not only will there be more old people, but there will be relatively fewer young people to support them. Professor Richard Lee of the University of California at Berkeley says this aging of the world has a great effect on economics. “Population aging increases the concentration of population in the older ages and therefore it is costly,” he said. Aging populations consume (消耗) more and produce less. Wit如图，甲、乙为两座建筑物，它们之间的水平距离BC为30日，在A点测得D点的仰角，在B点测得D点的仰角为，测得甲、乙这两座建筑物的高度分别为　　米．A. ，30 B. 30， C. ，30 D. ，