Read the following article and answer the question. Article: For thousands of years humans have used herbs , roots, and other things from nature to cure disease. Scientists looking for new cures often visit traditional healers. They find out what plants are used in healing. Then they look at what the plant contains. They find the active ingredient , which is the chemical in the plant that works to heal. Many of the drugs we use, including aspirin, have come from these natural medicines. In recent years, many people in the US have taking what are legally known as dietary supplements . They may be vitamins or minerals. They may be herbs that help with health problems, such as depression. Some of them are teas. Others are capsules or tablets. They are sold in natural food stores, drug stores, and groceries. Most people think that because these products are natural they are also safe. This is not always true. There are many plants that are poisonous. There are others that have bad side effects. The New England Journal of Medicine publishes medical research. A recent edition called for the government to _ dietary supplements. Several doctors wrote to the Journal and told of bad experiences people have had with dietary supplements. Dietary supplements aren't tested in the same way as drugs. There is not always proof that the supplement really does what it is supposed to do. There is also a problem with purity of the supplements. Supplements have been found to contain things that are bad for the body. For example, the California Department of Health found that of 260 herbs grown in China, one third contained heavy metals. These are minerals such as lead and mercury . They are very poisonous, even in tiny amounts. Question: The problem with dietary supplement may be in that _ . Answer:
Select from: [I] the supplements can't be tested like drugs; [II] the supplements may contain something useless; [III] the supplements may be polluted seriously; [IV] there is no clear evidence that the supplement does work well;
Answer:
[IV]