A can do a piece of work in 4 hours; B and C together can do it in 3 hours, while A and C together can do it in 2 hours B atone for doing in:

To find B's time, calculate individual work rates. A's rate is 1/4. (A+C)'s rate is 1/2, so C's rate is 1/2 - 1/4 = 1/4. (B+C)'s rate is 1/3, so B's rate is 1/3 - 1/4 = 1/12. Therefore, B alone takes 12 hours to complete the work. Options A, B, and D are incorrect calculations.