For which of the following equations does NO integer solution exist?
14x+21y=714x + 21y = 714x+21y=7
12x+18y=612x + 18y = 612x+18y=6
12x+18y=712x + 18y = 712x+18y=7
10x+15y=510x + 15y = 510x+15y=5