Marie Curie was the recipient of two Nobel Prizes—a shared prize in physics and an individual prize in chemistry. Her chemistry prize was for discovering two new elements, polonium and radium. For what did she, her husband, and Henri Becquerel receive the physics prize?

-The discovery that radioactivity is an extensive property that can be measured precisely.
-The discovery that certain substances can phosphoresce.
-The discovery that radioactivity is caused by the decay of unstable atomic nuclei.
-The discovery that radioactivity can ionize gas in a chamber.