A、About 30 years ago. B、In 1977. C、In the 1990s. D、Only in the past three years. A

In urban areas, car produces thousands of tones of lead in their exhaust fumes. In some large cities, the amount of lead in the air and soil is more than a thousand times than the natural level.
   For instance, in Singapore lead can affect the children in many ways. The most obvious way is through the exhaust fumes of cars. This means that people who live near busy roads, and especially those on the lower floors are most at risk. It is quite likely that the whole central part of Singapore is polluted by lead from this source. Another source is lead water pipes. Because it flows through the pipes, the water dissolves lead and carries it to household taps. In industrial waste lead is also present. After flowing into the sea, the lead is absorbed by fish, crabs which are eaten by human beings.
   Slow poisoning by lead affects the brain too. It makes the victim inactive. He becomes irritable and may even become violent. Lead attack the bones of young people and upsets their nervous system. Victims are far more likely to daydream than other children. And they have great difficulty concentrating on anything for very long. One expert said that she had taught children in the center of city. in the suburbs and in the countryside. She found that rural children who are less affected by lead had a better attitude to their studies and were easier to teach and control.
   Only about three decades ago, doctors began to discover just how dangerous lead can be. In many countries, governments are now urged to ban the use of petrol containing lead. In the USA, there were 2700 cases of severe lead poisoning in 1970, but this fell to 300 in 1977 when more areas used lead-free petrol.
Questions:
23.In Singapore children can, be affected by lead in many ways which of the following is NOT mentioned in the talk?
24.Which of the following is not the symptoms when the brain is affected?
25.When did people discover the great danger of lead, according to this talk?
26.How can we prevent the lead pollution?