Which statement best describes a redox reaction?

• A. A precipitate forms when two aqueous ionic solutions are mixed.
• B. Electrons are transferred between species, so oxidation and reduction occur simultaneously.
• C. A reaction occurs in which $H^{+}$ is transferred from an acid to a base without changing oxidation numbers.
• D. A compound breaks into simpler substances with no change in oxidation states.

Answer: (B)