A、They are dangerous to the health of animals and people. B、They can interfere with the ecological balance of the environment. C

It is clear that some chemicals can damage the health of animals and humans. However, this is not the only problem that can be caused by the careless use of chemicals. Chemicals can also disturb the ecological balance of the environment. If the ecological balance is disturbed, the consequences can be extremely serious.
    The history of DDT illustrates the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer to many problems. It would control insects that caused billions of dollars of damage to crops every year. Governments permitted and encouraged the use of DDT. Farmers in many countries began to spray it on their crops. The immediate results were good: damage to crops went down, and profits went up. However, the chemical had effects which the scientists did not predict. First, it also killed insects which were the natural enemies of the harmful insects and which were therefore beneficial to farmers. Second, and perhaps worse, DDT did not kill every harmful insect. A few insects had natural resistance to the chemical. They survived and multiplied. In a few years there were large numbers of insects which were not affected by DDT, and there were fewer insects which could act as natural controls on these new "super-insects". Finally, it became clear that DDT was not solving the insect problem. In fact, it was making the problem worse. It then became necessary to find a second cure for the effects of the first.
26. What are the problems with chemicals?
27. What’ s the attitude of governments for DDT?
28. What was the result of DDT?
29. What does the speaker mean by saying "It then became necessary to find a second cure for the effects of the first" ?
30. What does this passage mainly discuss?