Atomic mass is found by adding the number of protons and neutrons together. The atomic number tells you how many protons an element has. So since this element has 10 protons and an atomic mass of 20, there are 10 neutrons in this element.
protons and neutrons are in the nucleus electrons surround the atom and i have no idea what positrons are i just know they arent in an atom so your answer is B and C
