Astronaut Mark Kelly and his crew traveled to the International Space Station in the space shuttle. The shuttle burned off all remaining fuel before entering Earth's atmosphere. When the shuttle returned to Earth it landed at a speed of approximately 354 km/hr or 220 mi/hr, about 20 times the landing speed of a commercial aircraft. What accounts for the shuttle's speed upon landing?

A)
Earth's atmosphere

B)
initial thrust of take-off

C)
angle of approach upon landing

D)
gravitational potential energy