A sector is cut from a circle of radius 21 cm with central angle $150^\circ$. What is the length of its arc? Take $\pi=\dfrac{22}{7}$.

Arc length of a sector is $l=\dfrac{\theta}{360}\cdot 2\pi r$. Substituting $\theta=150^\circ$ and $r=21$ cm gives $l=\dfrac{150}{360}\cdot 2\cdot\dfrac{22}{7}\cdot 21=55$ cm. **Option D would correspond to taking too large a fraction of the full circumference.**