The Curies took pure uranium and a uranium-containing substance. When they measured the amount of radioactivity, there was more in the uranium-containing substance than the pure uranium. Which idea did the Curies most likely develop?

If the uranium-containing substance is more radioactive than pure uranium, the uranium alone cannot explain all the measured radioactivity. The Curies most likely concluded that the substance contained another radioactive material, supporting the idea that there are more radioactive substances than uranium. Mixing substances does not itself magnify radioactivity, and the observation does not show that uranium’s half-life changed.

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