A metre rule that is uniform and has a mass of $100\,\text{g}$ is held up by a pivot at the $40\,\text{cm}$ mark and by a string attached at the $100\,\text{cm}$ mark. The string goes over a frictionless pulley and supports a mass of $20\,\text{g}$, as shown in the diagram. At which mark on the rule should a $50\,\text{g}$ mass be hung so that the rule is in balance?
- A$4\,\text{cm}$
- B$36\,\text{cm}$
- C$44\,\text{cm}$
- D$64\,\text{cm}$