Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Plastic is a material we use every day. Plastic was first made more than 100 years ago from parts of plants. Plastic is now made from oil, coal and natural gas. We are using up these things so fast that the Earth's supplies may run out. Because of this, scientists are investigating new ideas for making plastic from plants such as sweet potato, bamboo and flax.

Things made from plastic can be useful for people but bad for the planet. Some kinds of plastic can last for a long time without wearing out, and can be difficult to get rid of when they are not needed. They can stay in rubbish dumps for hundreds of years.

Recycling and reusing arc good ways to solve the problem of unwanted plastic. Recycled waste materials can be used again to make new products. Some kinds of plastic can be melted down and used to make new things such as bags and bottles; others can be made into fibers for clothing. Plastic bottles can be refilled many times, rather than thrown away once they are empty. Unwanted plastic goods such as CDs and toys can be sold or given away to charity shops.

According to the passage, things made from plastic are harmful to the environment because _______.