Task: Question Answering
Input (English): Problem: a certain telephone company offers two plans, a and b. under plan a, the company charges a total of $ 0.60 for the first 9 minutes of each call and $ 0.06 per minute thereafter. under plan b, the company charges $ 0.08 per minute of each call. what is the duration of a call, in minutes, for which the company charges the same amount under plan a and under plan b ?
Options: a. 3, b. 9, c. 15, d. 21, e. 30
Output (English): a