Problem: in an electric circuit , two resistors with resistances 8 ohms and 9 ohms are connected in parallel . in this case , if r is the combined resistance of these two resistors , then the reciprocal of r is equal to the sum of the reciprocals of two resistors . what is r value ? 
Options: ['a ) 17', 'b ) 45 / 78', 'c ) 72 / 17', 'd ) 17 / 72', 'e ) 72']
c