The radioactive isotope caesium $^{{137}}_{{55}}\text{Cs}$ has a half-life of 30 years. If the starting mass is $30\,\text{g}$, what mass of the isotope is still present after 90 years?
- A$10.0\,\text{g}$
- B$7.50\,\text{g}$
- C$3.75\,\text{g}$
- D$1.25\,\text{g}$