The diagram represents a uniform metre rule. It is supported on a pivot at its centre. A force of $4.0\,\text{N}$ acts vertically downward on the rule at the $5\,\text{cm}$ mark. A string holds the rule at the $30\,\text{cm}$ mark. The rule is balanced in equilibrium. What upward force does the string apply to the rule?
- A$0.67\,\text{N}$
- B$4.0\,\text{N}$
- C$6.0\,\text{N}$
- D$9.0\,\text{N}$