Question

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?
there are more radioactive substances than uranium.
radioactivity is magnified by mixed substances.
the date of the material varied between the pure substance and mixed substance.
the half - life of uranium in a mixed substance is less than pure uranium.

Explanation:

Since the uranium - containing substance had more radioactivity than pure uranium, it implies there are other radioactive substances present besides uranium in the uranium - containing substance. Radioactivity isn't magnified by mixed substances in this context, date is not relevant to radioactivity levels shown here, and there's no indication of half - life differences from this experiment.

Answer:

There are more radioactive substances than uranium.