Task: Question Answering
Input (English): Problem: the number of people who purchased book a is twice the number of people who purchased book b . the number of people who purchased both books a and b is 500 , which is twice the number of people who purchased only book b . what is the number of people z who purchased only book a ? 
Options: a ) 250 , b ) 500 , c ) 750 , d ) 1000 , e ) 1500
Output (English): d