21. The escape velocity at the surface of Earth is approximately 10 km/s. What is the escape velocity for a planet whose radius is 4 times and whose mass is 100 times that of Earth?
(A) 0.4 km/s
(B) 2 km/s
(C) 50 km/s
(D) 250 km/s
(E) 4000 km/s
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統計: A(0), B(0), C(1), D(0), E(0) #3384055
統計: A(0), B(0), C(1), D(0), E(0) #3384055