Problem: consider a quarter of a circle of radius 20 . let r be the radius of the circle inscribed in this quarter of a circle . find r . 
Options: ['a ) 20 * ( sqr 2 - 1 )', 'b ) 8 * ( sqr 3 - 1 )', 'c ) 4 * ( sqr 7 - 1 )', 'd ) 12 * ( sqr 7 - 1 )', 'e ) none of these']
a