A 120 m trapping net stretched out into a 3-sided rectangular corral is used to front a flowing river. What value of widths (w), being parallel to the riverbanks, will provide the corral with largest capturing area?

A. w = 15 m

B. w = 20 m

C. w = 25 m

D. w = 30 m

E. w = 35 m

I'm stuck as to how the author came up with an answer for this one. Thanks in advance!