If you just added the masses of the 3 isotopes up and divided by 3, what would the answer be?

it's c. elements are placed in rows based on how may valencies electrons have.

a). in this case, dh> 0 and sd> 0. if t becomes larger, the term "-tds" will become more negative. at some point, it is possible that -dts will be more negative than the dh is positive, and the reaction will become spontaneous. this is probably the correct answer, but you should look at all of the other possibilities and make sure of that.