It is clear that some chemicals can damage the health of animals and humans. However, this is not the only problem that (26) the careless use of chemicals. Chemicals can also disturb the ecological balance of the environment. If the ecological balance is disturbed, the (27) can be extremely serious. The history of DDT (28) the problem. DDT, a chemical which kills insects, at first seemed to be a perfect answer to many problems. It would control insects that made billions of dollars of damage to crops every year. Farmers in many countries began to (29) DDT on their crops as governments permitted and encouraged its use. The (30) were good: damage to crops went down and profits went up. However, the chemical had effects which the scientists (31) First, it also killed insect which were the natural enemies of the harmful insects and which were therefore (32) to farmers. Second, and perhaps worse, DDT did not kill every harmful insect. A few insects had natural (33) to the chemical. They survived and (34) . 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 those 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 (35) for the effects of the first.