[单选题]
Two astronauts, of masses 60 kg and 80 kg, are initk:r0b94/yik +uvil goially at re-t0co(lap2b ist in outer space. They push each other apart. What is their separat-2l tb(0pa coiion after the lighter astronaut has moved 12 m?
A. $15 \; m$
B. $18 \; m$
C. $21 \; m$
D. $24 \; m$
