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A
At the rear
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B
The heaviest trailer should be FIRST (directly behind the tractor), with trailers in decreasing weight order toward the rear — this improves stability and reduces the severity of rearward amplification
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C
In the middle
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D
Weight order doesn't matter
Why this is the answer
TRAILER WEIGHT ORDER IN DOUBLES/TRIPLES: Place the HEAVIEST trailer FIRST (closest to the tractor), with trailers arranged in DECREASING weight toward the rear. WHY: A heavy trailer at the rear amplifies the 'crack-the-whip' effect and increases instability and rollover risk; a lighter trailer at the rear swings less violently and is more stable; keeping more weight forward improves handling and traction; STABILITY: The combination is most stable with weight distributed heaviest-to-lightest front-to-rear; LOADING within each trailer also matters — distribute weight properly, keep the center of gravity low, secure the cargo; REARWARD AMPLIFICATION: Since the rear trailer experiences the most amplification, having it be the lightest reduces the danger; REGULATIONS/PRACTICE: This loading principle is part of safe doubles/triples operation; drivers and loaders should arrange trailers by weight; an improperly ordered set (heavy trailer at the rear) is more likely to roll over or lose control in a maneuver; this weight-ordering principle is tested on the doubles/triples endorsement and is an important safety practice for multi-trailer combinations.
Source: FMCSA CDL Manual — Doubles/Triples, Trailer Weight Order