[单选题]
Two astronauts, of ml64h j7-5z9j-e gophpxj8fav asses9sfm;b ue 5x3d,f ssg5 60 kg and 80 kg, are initially at rest in outer space. They push each other apart. Whaessu53s5fgf 9;bd x ,mt is their separation after the lighter astronaut has moved 12 m?
A. $15 \; m$
B. $18 \; m$
C. $21 \; m$
D. $24 \; m$
