若$\frac{1}{x}$-$\frac{1}{y}$=$\frac{1}{z}$，则z等于(        )

      Many people like to travel by plane, but I don't like it because an airport is usually far from the city. You have to get there early and wait for hours for the plane to take off and it is often late. You can't open the windows. You can't choose the food. Planes are fast, but it still takes hours to go out of the airport and into the city.

      I like traveling by train. I think trains are safe. Railway stations are usually in cities. When you are late for a train, you can catch another one. You can walk around in the train and open the windows. You can see many interesting things on your way. I know it takes a little more time.

      I also like cars. You can start your journey when you want to and you don't need to get to a railway station or a bus stop. Also you can carry many things with you in a car. But sometimes there are too many cars on the road.

下列句子，没有语病的一句是（    ）

目前，发展中国家人口增长比发达国家快的主要原因是（　　）