Every time materials undergo physical or chemical changes..a. a new substance is always formed. b. the total amount of matter always increases. c. the total amount of matter always decreases. d. the total amount of matter always remains the same.

The answer is D 

Hope it helps

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.