Final answer:
An apex locator aids in determining the proper working length of a canal accurately.
Step-by-step explanation:
The correct answer is B. Apex locator. An apex locator is a device used in endodontics to accurately determine the working length of a root canal. It measures the distance from the coronal reference point to the apical constriction using electronic signals. This helps in ensuring that the canal is thoroughly cleaned and filled during root canal treatment.
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