The most common form of calcium has 20 protons, 20 neutrons, and 20 electrons. Which of the following elements would be an isotope of calcium?

• A. An atom with 21 protons, 20 neutrons, and 21 electrons
• B. An atom with 20 protons, 20 neutrons, and 18 electrons
• C. An atom with 21 protons, 21 neutrons, and 21 electrons
• D. An atom with 20 protons, 21 neutrons, and 20 electrons