oh ok those are the better choices. well a physical reaction means something changed in the property itself. so we can take off B and C. a chemical reaction means the property of the item changed completely and stayed that way. so im going with D.
The answer is actually B, since rusting is an obvious chemical reaction which cannot be undone. A is just fact, C can be undone by cooling, and D can be undone by splitting the molecules apart. So the answer is B.
