Read the passage and choose the best answer (A, B, C or D) to complete each question.

Humans have struggled against weeds since the beginning of agriculture. The global need for weed control has been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused. Toxic compounds threaten animals and public health when they are present in food plant, ground water, and drinking water. They also harm workers who apply them.

In recent years the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world’s weed problems. Therefore, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms.

The biological agents now in use are harmless to humans, environmentally friendly, and offer the benefit of specificity. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill almost all the plants they come in contact with, sparing only those that are naturally resistant to those that have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after that no added applications are needed. Chemicals, typically, must be used several times per growing season.

What is the passage primarily about?