Task: Question Answering
Input (English): Problem: an alloy of copper and zinc contains copper and zinc in the ratio 9 : 6 . another alloy of copper and zinc contains copper and zinc in the ratio 6 : 9 . in what ratio should the two alloys be mixed so that the resultant alloy contains equal proportions of copper and zinc ? 
Options: a ) 1 : 5 , b ) 1 : 4 , c ) 1 : 3 , d ) 1 : 1 , e ) 1 : 2
Output (English): d