The speed of a subway train is represented by the equation z=s^2+2s 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 5 seconds than it was moving after 3 seconds?	Choose the most appropriate option:
A) 4 B) 9 C) 15 D) 20 E) 63

Rationale: "given : z = s ^ 2 + 2 s for 0 ≤ s ≤ 7 z ( 3 ) = 3 ^ 2 + 2 * 3 = 15 z ( 5 ) = 5 ^ 2 + 2 * 5 = 35 therefore z ( 7 ) - z ( 3 ) = 35 - 15 = 20 km / hr option d"
Answer: D