B. 1,200 kg-m/s
Momentum is defined as the product of mass and velocity. So let's see what the momentum is for each spaceship.
Spaceship 1 has a mass of 300 kg and a velocity of 0 m/s, giving
300 kg x 0 m/s = 0 kg-m/s
Spaceship 2 has a mass of 300 kg and a velocity of 4 m/s, giving
300 kg x 4 m/s = 1200 kg-m/s
Since we're looking for their combined value, add the 2 figures together.
0 kg-m/s + 1200 kg-m/s = 1200 kg-m/s
So the answer is option B. 1,200 kg-m/s
