The Half-Life of Cobalt-60 Is $5.3$ Years, While That of Strontium-90 Is About 29 Years

The half-life of cobalt-60 is $5.3$ years, while that of strontium-90 is about 29 years. Suppose you have samples of both isotopes, and that they initially contain equal numbers of atoms of these isotopes. How will the activities (number of decays per second) of the samples compare?

A) The activities will be equal.
B) The activities cannot be compared without more information.
C) The activity of the strontium- 90 sample will be greater.
D) The activity of the cobalt-60 sample will be greater.