Task: Question Answering
Input (English): Problem: the ratio between the present ages of a and b is 7 : 3 respectively . the ratio between a ' s age 4 years ago and b ' s age 4 years hence is 1 : 1 . what is the ratio between a ' s age 4 years hence and b ' s age 4 years ago ? 
Options: a ) 3 : 4 , b ) 3 : 0 , c ) 9 : 1 , d ) 9 : 2 , e ) 3 : 9
Output (English): c