An adult passenger moves to the rear seat and leaves the belt undone because the trip is short. Which statement is correct?

Why D is correct

An unrestrained passenger goes on travelling at the speed the car was doing while the car itself stops, and whatever brings them to rest is the object they hit. Behind you that usually means the back of a seat somebody belted is sitting in, adding body weight to a crash load their restraint was never asked to manage. Modern restraint systems work alongside belts rather than instead of them: airbags, pretensioners and crumple zones all assume the occupant is secured. Leaving the belt off is not a private risk — it is delivered squarely to whoever sits in front.

Why the other options are wrong

A. Rear belts matter less, since the front structure absorbs most impact forces

Structural crush zones manage how the car decelerates, not how the people inside it do. Only the belt manages occupants, wherever in the vehicle they happen to be sitting.

B. Rear belts only begin to matter above roughly 40 mph

No threshold switches on at forty. Urban junction collisions mostly occur well below that figure and still deliver enough deceleration to move a body forward violently. Inside a car, everything not attached to it becomes a projectile travelling at whatever speed the car was doing at the moment of contact.

C. Modern restraint systems protect rear occupants adequately without belts

Supplemental systems are named for exactly that reason: they complete the belt's job rather than replace it. A passenger relying on airbags alone is relying on nothing designed for the purpose.

On the road

Two adults restrained in front, one adult behind them not. One junction, one impact, and a seatback taking a load it was never designed to manage. That passenger's decision belonged to somebody else too. Restraints are not a private choice about private risk: whoever sits ahead of an unrestrained passenger inherits whatever the restraint design was never built to absorb.

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