A. The metal spring in the flashlight acts as a wire. If it is replaced by a plastic spring, plastic is an insulator and does not conduct electricity, which will cause the circuit to break and the bulb will not emit light, so A is incorrect;
< p>BC, metal springs are wires and cannot convert elastic potential energy into electrical energy, nor can they convert electrical energy into light energy, so BC is incorrect;D. Metal springs are elastic and are conductors at the same time, so they can To prevent circuit breakage caused by poor circuit contact, D is correct.
So choose D