Answer:
matter is the correct answer.
The trains take <u>57.4 s</u> to pass each other.
Two trains A and B move towards each other. Let A move along the positive x axis and B along the negative x axis.
therefore,

The relative velocity of the train A with respect to B is given by,

If the train B is assumed to be at rest, the train A would appear to move towards it with a speed of 170 km/h.
The trains are a distance d = 2.71 km apart.
Since speed is the distance traveled per unit time, the time taken by the trains to cross each other is given by,

Substitute 2.71 km for d and 170 km/h for 

Express the time in seconds.

Thus, the trains cross each other in <u>57.4 s</u>.
Gases can be compressed, because they just take up the space surrounding them. The attractive forces between the particles in a gas are very weak, so the particles are free to move in random direction. They just move along until they collide, either with the walls of the container or with each other. Moreover, gases can be compressed because the particles are far apart and they have space to move into.
It doesn't mean that the climate is changing, it is probably just the morning dew.