下列措施中，能使蒸发变快的是(    )

Many people like to travel by plane , but I don't like it because an airport(飞机场) is usually far from the city. You have to get 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 they still take hours to go out the airport and into the city.
I like traveling by train. I think trains are safe. Train 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 train station or bus stop. Also you can carry many things with you in a car. But sometimes there are too many cars on the road.

有长24m的篱笆，一面利用围墙围成如图中间隔有一道篱笆的矩形花圃，设花圃的垂直于墙的一边长为x m，面积是s m²，则s与x的关系式是（　　）