Which of the following type of pollution is NOT mentioned in the conversation?

Listen to the conversation, mark each question as A, B, C or D according to what you hear.
W: Hello, Rob. Have you heard that air pollution has been a serious problem affecting some of the big cities in China?
M: Yes, or better known as smog. I’ve experienced this in Beijing and it’s not a nice thing to breathe in.
W: It certainly isn’t good for your health. Do you know which country has the city with the world’s worst air pollution, according to research by the World Health Organization?
M: Well, I think it is definitely China.
W: We’ll find out if you’re right later on. Let’s talk more about the pollution problem. Pollution is a word describing anything that damages something pure. So light pollution is when a pure dark sky is spoiled by street lights and lights in the city.
M: And noise pollution is when loud or annoying sounds like traffic noise spoil a quiet and peaceful environment. The smog is a form of air pollution.
W: Yes. Smog used to be a problem in other major cities like London, which was from factories and trains.
M: In Los Angeles, fumes—that’s the bad gases from car engines—and poor weather conditions used to cause poor air quality. But now, new laws and changes in technology mean the air in these cities is cleaner and safer. But why is it bad in China?
W: China is becoming more industrialized. More factories are being built, more electricity is needed, and air pollution often comes from coal-fired power stations. Of course, more people are driving cars.
M: So what can be done?
W: Chinese authorities are spending ￡180 billion on cleaning up the air. They’re closing down many power stations within the city that burn coal. That’s the biggest source of pollution. And they also limit traffic on the roads. Greener cars will be allowed on the roads. China is also going to use more renewable energy.
M: Well, of course, all these changes will take a long time. Oh, I nearly forgot. You didn’t tell me the answer to the question about which country has the city with the world’s worst air pollution. I thought it was China.
W: The answer is Iran.
M: Oh.