The correct answer is D. Neutrons are not emitted by radioactive substances.
Radioactive decay is a process in which an unstable atomic nucleus loses energy by radiation. The three main types of radiation emitted are alpha particles, beta particles, and gamma rays. Alpha particles are helium nuclei, which are made up of two protons and two neutrons. Beta particles are electrons or positrons, which are the antiparticles of electrons. Gamma rays are high-energy photons.
Neutrons are not emitted by radioactive substances because they are stable particles. They do not decay on their own, and they are not ejected from the nucleus during radioactive decay.
Alpha particles are the least penetrating type of radiation, and they can be stopped by a sheet of paper. Beta particles are more penetrating than alpha particles, and they can be stopped by a thin layer of aluminum. Gamma rays are the most penetrating type of radiation, and they can only be stopped by thick layers of lead or concrete.
Radioactive decay can be used to generate electricity, to sterilize medical equipment, and to date archaeological artifacts. It can also be harmful to human health, and it is important to take precautions to avoid exposure to radiation.