The diagram depicts a uniform metre rule. Its centre is the pivot point. A load of $4.0\ \text{N}$ is hung from the rule at the $5\ \text{cm}$ mark. The rule is supported by a string at the $30\ \text{cm}$ mark. The rule is balanced. What upward force does the string exert on the rule?
- A$0.67\ \text{N}$
- B$4.0\ \text{N}$
- C$6.0\ \text{N}$
- D$9.0\ \text{N}$