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 c who purchased only book a ?
Options: a. 250, b. 500, c. 750, d. 1000, e. 1500
Output (English): d