The correct option is D. The initial momentum of the blue and green train combined during the collision is 200 kgm/s.
Apply the principle of conservation of linear momentum as follows;
Pi = m1v1 + m2v2
where;
Pi = (50 x 4) + 30(0)
Pi = 200 kgm/s
Thus, the initial momentum of the blue and green train combined is 200 kgm/s.
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