A lead-screw with half nuts in a lathe, free to rotate in both directions has
Answer: D
A lead-screw with half nuts in a lathe, free to rotate in both directions had Acme threads. When it is used in conjunction with a split nut, as on the lead screw of a lathe, the tapered sides of the threads facilitate ready to engagement and disengagement of the halves of the nut when required.