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 3 hours. How long will B alone take to do it?  Choose the correct choice:
A) 12	B) 9	C) 8	D) 6	E) 4

Rationale: "a ' s 1 hour ' s work = 1 / 4 ; ( b + c ) ' s 1 hour ' s work = 1 / 3 ; ( a + c ) ' s 1 hour ' s work = 1 / 3 . ( a + b + c ) ' s 1 hour ' s work = ( 1 / 4 + 1 / 3 ) = 7 / 12 . b ' s 1 hour ' s work = ( 7 / 12 - 1 / 3 ) = 1 / 4 . therefore a alone will take 4 hours to do the work . e"
Answer: E