A、It is made mostly of iron. B、It is made mostly of steel. C、It is made mostly of plastic. D、It is made mostly of paperboard. C

Wind turbines are tall structures with large blades used to produce electricity. They are useful sources of low-cost, renewable energy.(3)But they can also be deadly to birds and bats. The turbines’ large blades turn quickly in the sky. Birds and bats sometimes fly into them and are killed.
    But a new type of wind generator may offer an answer to that problem. Mr. Yanez and his friend Raoul Martin built a small wind tunnel to experiment on their own. They built a large working model of the generator in a field near their workspace. They called the equipment Vortex. Mr. Yanez described how the Vortex generator works. It does not spin around. So it creates no risk to birds and bats.(4)It is made mostly of plastic and has very few moving parts. The generator also does not create noise.
3. Why are wind turbines deadly to birds and bats?
4. What is the Vortex generator made of?