Final answer:
The radioactive substance with a shorter half-life will be more radioactive for a shorter amount of time.
Step-by-step explanation:
The radioactive substance with a shorter half-life will be more radioactive for a shorter amount of time. The half-life of a radioactive substance is the time it takes for half of the nuclei to decay. So, if two substances have the same mass and volume, the one with the shorter half-life will have a higher rate of decay and will therefore be more radioactive for a shorter period.