Technician a Says That the Micrometer Operates on a Simple

Technician A says that the micrometer operates on a simple principle: The spindle has 20 threads per inch, so one revolution of the thimble will advance or retract the spindle 1/20 of an inch. Technician B says that spindle has 50 threads per inch, so one revolution of the thimble will advance or retract the spindle 1/50 of an inch. Who is correct?

A) A only
B) B only
C) Both A and B
D) Neither A nor B