Three Machines, Individually, Can Do a Certain Job in 2

Multiple Choice

Three machines, individually, can do a certain job in 2, 3 and 4 hours, respectively. What is the greatest part of the job that can be done in one hour by two of the machines working together at their respective rates?

A) 13/12
B) ¾
C) 5/6
D) 6/5