A rectangular field is to be fenced on three sides leaving a side of 30 feet uncovered. If the area of the field is 810 sq. feet, how many feet of fencing will be required? Choose the correct choice from the following choices:  A) 34  B) 40  C) 68  D) 84  E) none

Rationale: "explanation we have : l = 30 ft and lb = 810 sq . ft . so , b = 27 ft . length of fencing = ( l + 2 b ) = ( 30 + 54 ) ft = 84 ft . answer d"
Answer: D