A spaceship traveling in space with a constant velocity , now fires its engine and exhaust gases are ejected at a constant speed u relative to the spaceship. When the spaceship's speed becomes , its mass reduces to . The value of is closest to A、2.7 B、2.5 C、2.3 D、2.1
, now fires its engine and exhaust gases are ejected at a constant speed u relative to the spaceship. When the spaceship's speed becomes 
, its mass reduces to 
. The value of 
is closest to