Read the passage carefully and choose the best answer to each question out of the four alternatives.
The problem of water pollution by pesticides can be understood only in context, as part of the world to which it belongs- the pollution of the total environment of making. The pollution entering our waterways comes from many sources, radioactive wastes from reactors, laboratories, and hospitals; fallout from nuclear explosions; domestic wastes from cities and towns; chemical wastes from factories. To these is added a new kind of fallout the chemical sprays applied to crop lands and gardens, forests and fields. Many of the chemical agents in this alarming melange initiate and augment the harmful effects of radiation, and within the group of chemicals themselves there are sinister and little- understood interactions, transformations, and summations of effect.
Even since the chemists began to manufacture substances that nature never invented, the problems of water purification have become complex and the danger to user of water has increased. As we have seen, the production of these synthetic chemicals in large volume begin in the 1940’s. It has now reached such proportion that an appalling deluge of chemical pollution is daily poured into the nation’s waterways. When inextricably mixed with domestic and other wastes discharged into the same water, these chemicals sometimes defy detection by the methods in ordinary use by purification plants. Most of them are so complex that they cannot be identified. In rivers, a really incredible variety of pollutants combine to produce deposits that sanitary engineers can only despairingly refer to as “gunk”.

Q:

The word ‘gunk’ in the last line refers:

• 1
  to the waste products deposited by sanitary engineers
• 2
  to the debris found in rivers
• 3
  to unidentifiable chemicals found in water
• 4
  to the domestic water supplies