The half-life of a radioactive isotope is $2.5$ minutes. This isotope is the only radioactive isotope present in a radioactive sample. From time $t = 0$ to $t = 7.5$ minutes, the mass of this isotope in the sample falls by $70\,\mu\text{g}$. What is the mass of this isotope at $t = 0$?
- A$75\,\mu\text{g}$
- B$80\,\mu\text{g}$
- C$210\,\mu\text{g}$
- D$560\,\mu\text{g}$