[单选题]
Two astronauts, of masses b kuwl+/v/)(c o6fs pl60 kg and 80 kg, are inbz2)yq0ns2 pu;j-liboox k* +itially at rest in outer space. They push each other apart. What is their separation after the lighter astronaut has movedzpo2jn;lbbx2 0u - qso)ky*+i 12 m?
A. $15 \; m$
B. $18 \; m$
C. $21 \; m$
D. $24 \; m$
