What was one important discovery that the curies made?
a material’s radioactivity cannot be changed by altering the material physically.
a material’s radioactivity can be weakened or intensified by changing it chemically.
radioactive materials generally contain radium or polonium.
radioactive materials are found only in the form of salts.

A material’s radioactivity cannot be changed by altering the material physically.

Explanation:

Marie Curie studied the radiation of all compounds containing the known radioactive elements. She discovered that:

- The strength of uranium radiation can be exactly measured;

- The intensity of radiation is proportional to the amount of the radioactive element;

- The ability to emit radiation depends only on the interior of the atom, so it doesn't depends on the material physically.