Atrain travels 250 km westward from carthage to johnson city. the train arrives 2.5 hours after it left. what was the average velocity of the train between the two stations? a) 2.5 km/h west b) 25 km/h west c) 100 km/h west d) 250km/h west

Independent variable is something that is changed in a experiment. 

An atom's smaller negative particles are at a distance from the central positive particles, so the negative particles are easier to move. 

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero. 

Answerinternal combustion engine fuel;