16 A rocket of mass \( M \) when empty carries a mass \( M \) of fuel. The rocket and fuel travel at speed \( v \). The engine of the rocket is fired and all of the fuel is expelled. The speed of the rocket increases to \( 2 v \). What happens to the kinetic energy of the rocket? A It doubles. B It halves. C It increases by a factor of four. D It stays the same. C is the right answer. But how?