A, B and C can do a piece of work in 7 days, 14 days and 28 days respectively. How long will they take to finish the work, if all the three work together?  Select.
A) 4
B) 9
C) 2
D) 11
E) none

Rationale: "1 / 7 + 1 / 14 + 1 / 28 = 7 / 28 = 1 / 4 all three can finish the work in 4 days answer : a"
Answer: A