You will be given a definition of a task first, then some input of the task.
In this task, you need to answer the given multiple-choice question on geometry. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: the perimeter of a rhombus is 120 feet and one of its diagonal has a length of 40 feet . find the area of the rhombus .
Options: ['a ) 100 sqrt ( 5 ) ft . sq', 'b ) 400 sqrt ( 5 ) ft . sq', 'c ) 500 sqrt ( 5 ) ft . sq', 'd ) 200 sqrt ( 5 ) ft . sq', 'e ) 150 sqrt ( 5 ) ft . sq']
Output:
b