Instructions: Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.
Input: Problem: the speed of a subway train is represented by the equation z = s ^ 2 + 2 s for all situations where 0 s 7, where z is the rate of speed in kilometers per hour and s is the time in seconds from the moment the train starts moving. in kilometers per hour, how much faster is the subway train moving after 7 seconds than it was moving after 5 seconds ?
Options: a. 4, b. 28, c. 15, d. 48, e. 63
Output:
b