This was a question on an online homework for a modern physics class. I had two chances to answer it, but I was wrong both times, so I'm simply curious what the answer is. 1. The problem statement, all variables and given/known data 3. The attempt at a solution (a) Quarks attracted to one another is the strong force (b) A stable nucleus of 4 protons (Beryllium) and 5 neutrons is the strong force (c) Collapse of a star into a black hole is gravitational force (d) Beta decay of 139 Ba is the weak force (e) A rubber band stretching but not breaking... must the the electromagnetic force? I answered (d) and then (b). Is it really the rubber band?