Correct
磨杵成针
/ mó chǔ chéng zhēn /
Strokes
Definition
to grind an iron bar down to a fine needle (idiom); fig. to persevere in a difficult task/to study diligently
Example
Definition
grind an iron pestle into a needle―any difficult task can be accomplished as long as one persists to the end; little strokes fell great oaks