1 It may be hard to believe. 2 But light was once a rare and inviolate commodity. 3 Our ancestors invented willful ways to harness natural light; burning lumps of tallow or animal fat, capturing fireflies in glass jars, and even threading wicks down the throats of a bird called the storm petrel.4 Most of these methods were messy, inconvenient, and costly. 5 So it was fortunate when kerosene became popular. 6 Whale oil was another way to get light, but whaling was a dangerous and deadly occupation. 7 The innovation of the electric light bulb represented another huge leap in convenience and cost-effectiveness. 8 While light is not scarce today; in fact, we often have the opposite problem. 9 Light pollution, or excessive artificial light, which can keep city denizens from ever seeing the stars or appreciating true darkness.

In Sentence 7, what word should replace “innovation”?