Questions 11—13 are based on the passage you have just heard. You now have 15 seconds to read Questions 11—13. Why does the speaker cite the example of graffiti in the London underground
A．To show that mindless graffiti can provoke violence. B．To show that Londoners have a special liking for graffiti. C．To show that graffiti, in some cases, can constitute a crime. D．To show that graffiti can make the environment more colorful.