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 correct answer:
A) 4 B) 9 C) 15 D) 20 E) 63
Given: z=s^2+2s 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= 
<gadget id="calculator">35 - 15</gadget>
<output>20</output>
20 km/hr   Option 20
<result>D</result>
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?