Nat lived in a small town in England. He always stayed in England for his holidays, but last year he thought, “I've never been to other countries. All my friends go to Spain （西班牙）, and they like it very much,so this year I'm going there, too.” First he went to the capital, Madrid （马德里）, and stayed in a hotel for a few days. On the first morning he went out for a walk. In England people drive on the left （左） , but in Spain they drive on the right （右）. Nat forgot about this , and while he was crossing a busy street, a bicycle knocked （撞） him down.

　　Nat lay （躺） on the ground for a few seconds and then he sat up and said, “Where am I ?”

If we are driving a car or crossing a street, we will stop when the traffic lights turn red. Why do we use red rather than other colors?
As we know, scattering (散射) happens when light goes through the air. For the same media, if the wavelength (波长) is long, the scattering will be short. If the wavelength is short, then the scattering will be long.
Of all light we can see red has the longest wavelength, so the scattering is the weakest. That means the red light travels far. In fact, it travels farther on rainy days.
Using the red light can help drivers in farther areas see the lights. It helps drivers slow down or stop in time. If drivers don’t see the red light until they are close, they may not be able to stop the car in time. An accident may happen. Red can also excite people. We can act more quickly when we see the color red.
In a word, red light helps drivers stop in time and help prevent accidents.
【小题1】______ has the longest wavelength of all light we can see.

Go down this street, then ________ at the second crossing.　　　

[　　]

A．turn right 　　　    B．turn in

C． turn on 　　　　　  D． turn off