Answer:
B -- Cobalt and iron
Explanation:
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The train is moving at 50 m/s and Emma is walking down the aisle with 1 m/s speed in the same direction of train. The relative velocity of Emma with respect to other passengers pf the train would be 1 m/s. This is because, the train is not moving relative to them and only emma is moving at 1 m/s. If a person observes from outside, Emma would have (50 +1) m/s = 51 m/s velocity.
relative velocity when two objects are moving in same direction as oberved from outside observer:

relative velocity when two objects are moving in the opposite directionas oberved from outside observer:

Because of Surface tension
Force = change of momentum / time taken
Force = (90x3)/0.6