Which principle explains why it is necessary to have safety belts in cars

The main functions of  seat belt is to keep the occupant in a more static motion despite a sudden stop or change in momentum.

A car moves with inertia, which is an object's tendency to move until something works against the motion of that object. When the vehicle hits something, or is hit by something and  that makes the inertia change.

what are the choices?

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

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