If you add water to an industrial-strength hydrogen peroxide solution, you will likely dilute the concentration of hydrogen peroxide, resulting in a weaker solution. This would not produce a new substance, rocket fuel, or a bleaching agent. It is also unlikely to result in an explosion or antiseptic, as industrial-strength hydrogen peroxide solutions are typically much stronger than the 3% solution used for these purposes. Therefore, the best answer choice would be "C. a new substance." However, it is important to note that adding water to an industrial-strength hydrogen peroxide solution could potentially be dangerous and should not be done without proper safety precautions and expert guidance.