[单选题]
Two astronauts, of massesrqi0s)ab n4;i 60gci1 ;i 45wiqq kg and 80 kg, are initially at rest in outer space. They push each other apart. What is their separation after the lighter astriiqi 4wq1g c;5onaut has moved 12 m?
A. $15 \; m$
B. $18 \; m$
C. $21 \; m$
D. $24 \; m$
