In steel erection, to protect workers below from falling objects, all materials and tools at heights must be:

解析

All materials, equipment and tools not in use at height must be secured against accidental displacement, and loose items such as bolts, nuts and washers must be kept in closed containers. The rule is about the object moving, which is what each wrong answer misses. Lowering everything at the end of the shift (b) does nothing about the bundle kicked off a beam at ten in the morning. A two-foot setback from the edge (c) is the spoil-pile rule from section 1541(j)(2) of the excavation orders transplanted to a steel frame, where the deck is open on every side and there is no meaningful edge to set back from. And hard hats (d) are genuinely required for exposed workers below, as a second layer; a hat is rated for impact from a small object and does not make an unsecured load safe to leave.