Task: Question Answering
Input (English): 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 (English): b