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:
Ambient divers are, unlike divers who go underwater in submersible vehicles of pressure resistant suits, exposed to the pressure and temperature of the surrounding ambient water. Of all types of diving, the oldest and simplest is free diving. Free divers may use no equipment at all, but most use a face mask, foot fins, and a snorkel. Under the surface, free divers must hold their breath. Most free divers can only descend 30 to 40 feet, but some skilled divers can go as deep as 100 feet.
Scuba diving provides greater range than free diving. The word scuba stands for self-contained underwater breathing apparatus. Scuba divers wear metal tanks with compressed air or other breathing gases. When using open-circuit equipment, a scuba diver simply breathes air form the tank through a hose and releases the exhaled air into the water. A closed-circuit breathing device, also called a rebreather, filters out carbon dioxide and other harmful gases and automatically adds oxygen. This enables the diver to breathe the same air over and over. In surface-supplied diving, divers wear helmets and waterproof canvas suits. Today, sophiticated plastic helmets have replaced the heavy copper helmets used in the past. These divers get their air from a hose connected to compressors on a boat. Surface-supplied divers can go deeper than any other type of ambient diver.
Question: What are the helmets that surface-supplied divers use today made from, according to the passage?
