Since the Industrial Revolution, people have been pouring toxic waste into the land, ranging from petroleum products to heavy metals, and it is starting to have adverse effects on planet Earth itself as well as human and animal life. There is some level of mercury in all fish tested in the United States, and heavy metals in soil are now affecting the food supply. In Brazil, abandoned gold mines have leaked mercury into the soil. This very toxic heavy metal is absorbed by the grass and then eaten by cows, which are then processed for human consumption.     Nature, however, has an amazing way of dealing with such toxins. There are certain plants that can be grown which will remove toxins from the ground in a process called "phytoremediation" that is very promising for making contaminated soil safe for humans. Scientists are working with farmers in Brazil to grow maize and canola plants around farms, which assimilate the mercury and gold that are in the soil. The plants are then burned to ash, and the heavy metals can then be extracted from the ash to pay for the phytoremediation process. Mustard greens and pumpkin vines have been used to clean up yards and factory sites. Sunflowers have been used to rid the soil of radioactive metals near nuclear plants.     There are a few drawbacks, or precautions, when contemplating phytoremediation for soil treatment. The plants need to thrive in the climate of the soil, and they can only remove toxins as deep as their roots grow. So, phytoremediation will not clean up contaminated groundwater. When considering using plants to clean up soil, be mindful not to introduce an invasive species that will take over and be hard to control. Cleaning soil with plants takes time and does not happen overnight, but it is a natural, cheap, and environmentally friendly solution.4Which statement best expresses a central idea in the passage? A. Farmers in Brazil are having a hard time because of soil contamination. B. Phytoremediation is an easy way to naturally extract gold from soil. C. Petroleum products have contaminated our soil and the beef industry. D. Using plants to clean soil is a natural, sound, and inexpensive process.