Very small tubes.

• A. nanotubes
• B. water cycle
• C. open water
• D. atoms

Answer: (A)

Very small tubes are nanotubes. The name comes from nano, meaning one billionth of a meter, and tube, describing the hollow cylindrical shape they have at that tiny scale. Carbon nanotubes, for example, are made by rolling carbon sheets into tubes, giving them remarkable strength and interesting electrical properties. The water cycle describes how water moves and changes form in the environment, open water is simply a body of water, and atoms are the tiny building blocks of matter; none of these are tubes at nanometer scales. So nanotubes correctly name the extremely small tube structures.