When a metal reacts with a nonmetal, the metal
a) lose electrons and form a positive ion
b) gain electrons and form a negative ion
c) gain protons and form a negative ion
d) lose protons and form a positive ion

You would be correct, boiling water does disinfect contamination.    the property of disinfecting means to kill  germs, bacteria and other pathogens or contamination  that might cause an infection or cause a person to become sickly. a couple methods of disinfecting are using soap, other cleaning supplies or by simply "boiling" the contaminate.  the reason why boiling water disinfects certain attributes is because some cells cannot survive the heat and pressure.

Ithink about 50 % of an original isotope's number of atoms remain