Workers on a leading edge 6 feet or more above a lower level must be protected by:

解析

Leading-edge work at 6 feet or more requires conventional fall protection: a guardrail system, a safety net system, or a personal fall arrest system. The alternatives in the wrong answers are real but conditional. A controlled access zone with a safety monitor (b) is available only where the employer has first demonstrated that conventional protection is infeasible or would create a greater hazard, and then only as part of a written fall protection plan. A warning line (c) belongs to low-slope roofing and is set back 6 feet, or 10 where mechanical equipment is in use, never 20. And a plan alone (d) is not a measure - the plan has to specify what will be done instead. On a California job site the equivalent trigger is section 1670(a): falls in excess of 7 1/2 feet.