-
A
Decoration only
-
B
Monitoring the area to the left and rear — vehicles in the left lane adjacent to the trailer, merging traffic, vehicles entering your blind zone, and the trailer's position and any cargo issues
-
C
Seeing directly behind the trailer (it cannot do this)
-
D
Checking the passenger side
Why this is the answer
A CMV driver relies heavily on mirrors because there is no rearward visibility through a solid-sided trailer. MIRROR SYSTEM ON A TRACTOR-TRAILER: (1) LEFT CONVEX MIRROR: Provides a wide-angle view of the area to the left and slightly behind the cab; shows left-lane traffic that may be alongside the trailer; essential for lane changes to the left; wide-angle distortion makes objects appear farther than they are; (2) RIGHT CONVEX MIRROR: Same function on the right side; covers the right side and right rear; right side has a much larger blind spot and this mirror is correspondingly more critical; (3) FLAT (REAR-VIEW) MIRRORS: Provide straight-back view along the side of the trailer; used for monitoring trailer position, detecting trailer tire issues, watching for fires or shifted cargo; (4) WIDE-ANGLE MIRRORS (WEST COAST STYLE): The large convex mirrors mounted high on tractor doors — provide the broadest view but with significant size distortion. MIRROR ADJUSTMENT: Before every trip, mirrors should be adjusted so the trailer's side is just visible in the inner edge of the mirror — this provides the reference point for judging gap. BLIND SPOTS: Even with properly adjusted mirrors, every CMV has blind spots: directly behind the trailer (cannot see any vehicle directly behind); directly alongside the cab (the 'right side' blind spot extending from the door back 20-30 feet); immediately in front of the cab. CHECK EVERY 5-8 SECONDS: Mirrors should be scanned in rotation continuously — don't fix attention on a single mirror.
Source: FMCSA CDL Manual, Chapter 3, Seeing, Mirrors