Answer: A is nucleus , B is protons ,C is neutrons and D is electrons.
Explanation: All matter, including mineral crystals, is made up of atoms, and all atoms are made up of three main particles: protons, neutrons, and electrons. As protons are positively charged, neutrons are uncharged and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass.