Which of the following statements is true?

natural energy resources are nonrenewable and are being depleted.
current fuels are not polluting the air.
mercury released into aquatic ecosystems comes from rock erosion.
the natural cycles of earth can clear up our current air and water pollution problems.

Red blood cells are responsible for transporting oxygen in the blood. people with sickle cell anemia were born with a defective gene that results in sickle-shaped red blood cells. individuals with the disease often suffer pain in their extremities due to the build-up of these cells causing blockages in the blood vessels. the shape of the blood cells also reduces the amount of oxygen transported in the blood.