18. Nickel is a chemical element with the symbol Ni, and with an atomic number of 28, its chemical series will be classified as metal, and also a Transition metal, from period 4 and group 10. Therefore letter A
19. Subatomic particles will have a few differences among them, one of these differences is mass, because protons and neutrons have a very similar mass, but both masses are way heavier than electrons, therefore letter B is correct