A rectangular public park has an area of 3,600 square feet. It is surrounded on three sides by a chain link fence. If the entire length of the fence measures 180 feet, how many feet long could the unfenced side of the rectangular park be?
Indicate all such lengths.
A. 30
B. 40
C. 60
D. 90
E. 120

After the solving the question, the check will fail for 60, if we consider 60 then it will become square... please tell how ETS provide the right solution for this question. Thanks