A spacecraft of mass 1000\,kg in deep space expels 20\,kg of gas backward at 500\,m\,s−1 relative to the spacecraft. Its initial velocity is zero and external forces are negligible. What speed does the spacecraft gain immediately after the gas is expelled?
a) 20m s−1
b) 9.8m s−1
c) 10m s−1
d) 71m s−1