The diagram depicts a uniform metre rule. It is pivoted at its mid-point. A downward force of $4.0\,\text{N}$ acts on the rule at the 5 cm mark. A string supports the rule at the 30 cm mark. The rule is balanced and in equilibrium. How large is the upward force produced by the string on the rule?
- A$0.67\,\text{N}$
- B$4.0\,\text{N}$
- C$6.0\,\text{N}$
- D$9.0\,\text{N}$