Read the following selection and answer the questions that follow. Mercury is a heavy metal. It occurs naturally in minerals and rocks. Throughout most of human history, natural sources released mercury into the air at a constant rate. More recently, however, human activity has accelerated the release of mercury. Burning fossil fuels such as coal releases mercury into the air. Eventually, it ends up in soils or in surface water. Microbes in the water convert mercury into methylmercury. The mercury gets concentrated in the tissues of predatory fish such as shark and swordfish. Large fish typically concentrate more mercury than small fish. People who eat fish with high methylmercury concentrations can suffer from muscle tremors, deafness, and poor concentration. Pregnant women and children are especially sensitive to its toxic effects. Learning disabilities and developmental delays are common in children who have been exposed to significant levels of methylmercury during development. Based on the health effects described above, methylmercury would be best classified as a (an)