A set of 10 new questions in each round. Question: ( 1 of 10 ) William can finish a work in 11 days. Richard can finish the same work in 12 days while Joseph can finish the work in 13 days. How long will it take to finish it if they work together? (A) 9 days (B) 12 days (C) 3 423/431 days or 3.981 days (D) 6 423/431 days or 6.981 days