[单选题]
Two astronauts, of masses 6stdz gpon12 77rf ;b:k0 kg and :96y t;t aoyvd80 kg, are initially at rest in outer space. They push each other apart. What is th;o96:vaty tdyeir separation after the lighter astronaut has moved 12 m?
A. $15 \; m$
B. $18 \; m$
C. $21 \; m$
D. $24 \; m$
