The Landmark NIOSH Studies on Back Belts Have Shown That

The landmark NIOSH studies on back belts have shown that:

A) mandatory use of back belts is more effective than voluntary use.
B) voluntary use of back belts is more effective than mandatory use.
C) the incidence of "self-reported back pain" is related to failure of employees to use back belts.
D) all of the above.
E) none of the above.