Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Metalloids are the smallest class of elements. (The other two classes of elements are metals and nonmetals). There are just six metalloids. In addition to silicon, they include boron, germanium, arsenic, antimony, and tellurium. Metalloids fall between metals and nonmetals in the periodic table. They also fall between metals and nonmetals in terms of their properties. Answer: elements
A:
Metalloids, metals, and nonmetals are all classes of what?