This means the device allows users to complete twice the amount of work with the same amount of effort used to complete a task without assistance.
Numbers are assigned to demonstrate the amount of work that can be completed using the same amount of effort. "2" denotes twice the amount of work, "3" triple the amount, and so forth.
B The number "2" describes the effectiveness of the machine.
C The number refers to the speed of the device in question.
D This means that the device requires twice the amount of effort to complete the same amount of work.