28. Calculating the working length of an endodontic instrument involves several steps. Which of the following steps should be last?
A. Measure the tooth on a good preoperative radiograph.
B. Measure the tooth from the end of the instrument to the end of the root, and then subtract 0.5 mm as a safety factor. C. Insert the instrument to the stop and take another radiograph.
D. Subtract 3.0 mm from the initial tooth measurement and position a stop at this length on the first instrument to be used.