Problem: in an electric circuit , two resistors with resistances m and n are connected in parallel . in this case , if p is the combined resistance of these two resistors , then the reciprocal of p is equal to the sum of the reciprocals of m and n . what is p in terms of m and n ? 
Options: ['a ) ( n - m )', 'b ) mn / ( m + n )', 'c ) ( nm )', 'd ) ( n - m ) / ( m + n )', 'e ) none of these']
b