A、Dense plantings compete light with weeds. B、Dense plantings compete air with seeds. C、Dense plantings compete nutrition with s

Consider the fact that crops generally produce several hundred seeds from each plant. By comparison,(19)each weed plant can produce tens or even hundreds of thousands of seeds. And some buried seeds can survive up to forty years or even longer. Eradicating weeds means that you have to remove all the seeds and roots so the plants will not grow back. But birds or the wind can reintroduce them to the land. A more common way to deal with weeds is to control them enough so that the land can be used for planting. Chemical weed killers or natural treatments like corn gluten can suppress weed growth. Dense planting of a crop can also act as a natural control. One of the most common methods for suppressing weeds is dense planting.(20)A dense, competitive crop that quickly shades the soil will help suppress many weeds. The seeds need light to grow, so blocking the sun will reduce weed growth. Animals like sheep or goats eat weeds, so they can provide a biological control. Insects and other organisms can also act as biological controls.(21)Preventing the spread of weeds is an important part of weed management. Farm vehicles should be kept out of areas with weeds. If that is not possible, then clean off the equipment and your shoes when leaving. Another way to make use of weeds is to compost them. Heat is produced in the process of making organically rich compost to improve soil. The heat will kill many weed seeds. The same is true of seeds that pass through animals that graze on weeds.
Questions 19 to 21 are based on the passage you have just heard.
19. How many seeds can each weed plant produce at most?
20. How can dense plantings suppress weeds according to the speaker?
21. How to prevent the spread of weeds?