A uniform metre rule with mass $100\text{ g}$ is held up by a pivot at the $40\text{ cm}$ mark and by a string at the $100\text{ cm}$ mark. The string runs 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$64\text{ cm}$