What is the mass number of deuterium? A. 2 B. 4 C. 3 D. 1

The correct answer is C. Deuterium has a mass number of 2. This means that the nucleus of a deuterium atom contains 2 protons and 1 neutron. The other options are incorrect. Option A is incorrect because the mass number of helium is 4. Option B is incorrect because the mass number of tritium is 3. Option D is incorrect because the mass number of hydrogen is 1.

Deuterium is a stable isotope of hydrogen. It is also called heavy hydrogen. It is a rare isotope, making up about 0.015% of all hydrogen atoms. Deuterium is used in nuclear fusion reactors, as a tracer in chemical research, and in some types of nuclear weapons.