A uniform metre rule is balanced on a pivot at the $50\,\text{cm}$ mark. A mass of $25\,\text{g}$ is put at the $30\,\text{cm}$ mark on the rule. What is the smallest mass that could be added to the rule so that equilibrium is restored?
- A$5\,\text{g}$
- B$10\,\text{g}$
- C$15\,\text{g}$
- D$25\,\text{g}$