What is the isotope with 50 protons, 68 neutrons, and 50 electrons?

The answer for #3 is c but i don't know why

Hydrogenation reactions, in which h2 and an "unsaturated" organic compound combine, are used in the food, fuel, and polymer industries. in the simplest case, ethene (c2h4) and h2 form ethane (c2h6). if 140 kj is given off per mole of c2h4 reacting, how much heat (in mj) is released when 12 kg of c2h6 forms?

The empirical formula of a compound is ch2o and its mass is 120 amu/molecule, what is its formula?