A pure substance made of only one kind of atom.

• A. water cycle
• B. element
• C. Periodic Table
• D. open water

Answer: (B)

The main idea here is what defines an element: a pure substance that contains only one kind of atom. Elements are the simplest building blocks of matter; they can exist as individual atoms or as molecules made of identical atoms (for example, O2 or N2), still consisting of only one type of atom. The other options describe things that aren’t pure substances: the water cycle is a process describing how water moves through the environment; the Periodic Table is a chart that lists elements, not a substance itself; open water refers to water, which is a compound made of hydrogen and oxygen atoms, so it contains more than one kind of atom. Therefore, a pure substance made of only one kind of atom is an element.